# Pages 26 through 46

## Page 5 – Add to 10 Ones DigitPage 6- Add to 10 Tens DigitPage 7 – Multiplying by 25Page 8 – Multiplying by 50Page 9 – Multiplying by 75Page 10 – Multiplying by 125Page 11 – Multiplying by 11Page 12 – Multiplying by 111Page 13 – Multiplying by 12Page 14 – Multiplying by 15Page 15 – Multiply by 16 2/3Page 16 – Multiplying by 101Page 17 – Multiplying by 1001Page 18 – Multiplying both over 100Page 19 – Multiplying both under 100Page 20 – Multiplying over/under 100Page 21 – Double/Half MultiplicationPage 22 – Double/Double DivisionPage 23 – Adding/Subtracting FractionsPage 24 – FOILPage 25 – Multiplying over/under square

#### 50 comments

1. Hi there. Thanks for the number sense video resources/tricks. Using different platforms for students to learn this year has been a new thing I have been searching for. Thanks so much.
Bryan Neal

• S.C. on April 9, 2015 at 12:30 am

Thank you for these awesome tricks. You are very thoughtful for posting these videos on top of being a teacher and teaching your own students. My score has increased magnificently. Thank you so much!

1. Your welcome, I’m going to start working on a Calculator book and video series sometime this summer.

• Lightning on May 29, 2015 at 10:02 pm

Hello. I was wonder if there was a trick for multiplying 43×34 or 67×76. Like a reflection trick. Thank you.

1. I don’t know of one although Ill spend sometime on the Algebra and see if i can find something.

• Math Rocks on April 26, 2016 at 11:50 pm

u can use foil

2. Just changed the formatting for the pages with videos. Each video now has its own page. I had a problem with the plugin I used for the page and had to change it a bit. Let me know if anyone sees mistakes.

• Adhav on October 10, 2015 at 1:52 am

Is there a shortcut to this problem?
If there is can you help me solve this problem.

9 2/15 – 5 4/5 = ?

Thank you

1. Not really for this one. Just crunch the numbers like normal.

• Math Rocks on April 26, 2016 at 11:50 pm

What I did when I saw that question on the MSNS was I did 4/5 and made that 12/15. Then, I subtracted 12/15 from 2/15. I did this by going up from 12 to 15 (add 3) and then I added 2 to get 1 and 2/15. I added 5/15 to 12/15 to get to (1 and) 2/15. 5/15 simplified is 1/3. Then, since I went UP 5/15 to solve for the fraction, I did 9-5, which is 4, but since I went UP to simplify the fraction, I subtracted 1 more time, to get 3 and 1/3 as my answer. This probably won’t even make sense to you, but if it does, I hope it works later in the future. It’s easier to explain it visually.

• Adhav on June 11, 2016 at 3:08 pm

Thanks

2. Another interesting way is to be OK with negative answers. 9-5=4 and 2/15 – 4/5 = 2/15 -12/15 = – 10/15 so you get 4 and – 2/3 or 3 and 1/3. Which is nice since the numbers stayed small.

• Adhav on June 11, 2016 at 3:09 pm

Thanks

• Jake on October 22, 2015 at 1:36 am

Is there an easy way to multiply a 3 digit number by another 3 digit number
Ex:
565*344

1. Are you doing Number Sense Tests? If so I think you are doing an estimation question. If that is the case you dont really care what the exact answer is. If I was doing this question you asked with estimation then is alot easier. 565*344 is really close to 560 * 333. which would be 560 *1/3 with some zeroes. Which is really easy.

• Mia hinojosa on November 9, 2015 at 2:09 am

Hey do you know the 33 1/3 trick I could not find it in your book?

1. 33 and 1/3 is basically 1/3 think of it as 1/3 and do the math then figure out how many zeroes to add.

• Math Rocks on January 6, 2016 at 1:20 am

Hello. On the Cross Timbers Number Sense Test on October 2015, there was a question (question 76) that I didn’t know any trick for. It wasn’t an estimation question either, so I was wondering how to do it. The question was, 143 x 133. Can you tell me how to solve this if there is a way to? Thanks.

1. I’ll give you a hint 143 is a multiple of 7.

• Math Rocks on January 23, 2016 at 3:15 am

Thanks. I saw it on page 28

• Adhav on January 11, 2016 at 1:16 am

is there a trick for multiplying 213 by 138?

• Math Rocks on March 1, 2016 at 12:11 am

I don’t know if this will help, but I found that 213 = 71 x 3, and 138 = 23 x 6, or 23 x 2 x 3.

1. I discovered those myself not to long ago. I think in later editions of the book I will include those. Thanks.

• Math Rocks on February 19, 2017 at 8:46 pm

you should round 138 to 140 and 213 to 200

• Math Rocks on April 20, 2016 at 1:17 am

Well, Flour Bluff has dominated once again. I’m from Fowler, and we got 3rd place. I can’t understand how Flour Bluff could get 5100 points! They’re so good, but next year our school will probably be moved down to 4A because there is a new school coming in. But anyways, I’ll get to the point. Mr. Gillespy, I have a question for you about the recent state test. How do you number 53, which states A set with 10 elements has how many 7-element subsets? BTW, I got 5th place in Number Sense!

1. Fowler did really good this year. You guys gave us a run for our money and should be extremely proud of your results. Having you guys in 5A definitely makes the contest more exciting.

Great job getting 5th at state I know that was incredibly hard to do. hopefully my book helped.

To answer your question about question 53, subsets with certain amounts are just variations of Combinations so just think of it as a 10 choose 7 combination which is 10!/ (7! x 3!)

• Math Rocks on April 25, 2016 at 3:54 am

Thanks Mr. Gillespy. Your book helped a lot too. I just have one more question. How do you solve this: The slope of a line with x-intercept of (5, 0) and a y-intercept of (0, â€“2) is?

1. What are you asking.? The slope or the equation of the line?

• Math Rocks on May 30, 2016 at 5:34 pm

slope

• Math Rocks on January 29, 2017 at 5:50 pm

never mind… I made a simple mistake while trying to do y1-y2/x1-x2

• Math Rocks on April 26, 2016 at 11:41 pm

We might not be in 5A next year, because there is a new high school coming in next year, and it will probably lower the population which might place us in 4A. Can you answer the question I asked you?
Thx

• Lightning on December 20, 2016 at 6:18 pm

Can u please make a calculator section tricks to ur website. I have a bit of trouble with it.

1. Someday I will get around to it. Not ready to do that just yet.

• Math Rocks on February 15, 2017 at 3:11 am

Hi Mr. Gillespy,

How do you do a problem like a times b/c. An example is 16 x 19/21. I know you can do it in your head but I was wondering if there was a fast way

• Math Rocks on February 15, 2017 at 3:13 am

Hi Mr. Gillespy,

How do you do a problem like a times b/c. An example is 16 x 19/21. I know you can do it in your head but I was wondering if there was a fast way

• Math Rocks on February 18, 2017 at 12:02 am

sorry for commenting the same comment two times, my computer was facing some problems and I think I double clicked the send button for the comment

1. This problem is actually pretty hard to explain but ill give it a try. the key is that the denominator is 21. Subtract 19 and 16 from 21. You get 2 and 5. Multiply those numbers.
That is your new numerator. Your whole number is like the old trick. Since 19/21 is less than 1 you subtract the difference from 16. So the final answer is 14 10/21 . This gets harder when you multiply the numbers and they are negative. Look for this trick in my new book.

• Jacob on November 9, 2017 at 12:54 pm

Thanks so much for making this book. Our school Vandeventer benefits from it a lot!

1. Great to hear. Have a great season.

• NK on February 8, 2018 at 11:15 pm

does any of this help if you are taking the PSAT 8/9?

1. Some of it. General Math and Calculator would help more.

• NK on September 9, 2018 at 2:20 pm

This will help for math and science team, right?

1. I hope…

• MP on September 29, 2018 at 3:29 pm

I was wondering if there was a trick for things like 1+2+3…+15=.

1. yep its in the book and videos.

3. So, the double/double division trick is basically that since you have doubled both numbers when you divide it it comes out to the same number as the original equation, right?

1. yep

• Smiley on January 29, 2019 at 1:39 am

This is really helpful math and science learned alot

• Cat on March 24, 2019 at 7:15 pm

Hi I like math

1. LOL nice troll.