Page 6- Add to 10 Tens Digit

Page 7 – Multiplying by 25

Page 8 – Multiplying by 50

Page 9 – Multiplying by 75

Page 10 – Multiplying by 125

Page 11 – Multiplying by 11

Page 12 – Multiplying by 111

Page 13 – Multiplying by 12

Page 14 – Multiplying by 15

Page 15 – Multiply by 16 2/3

Page 16 – Multiplying by 101

Page 17 – Multiplying by 1001

Page 18 – Multiplying both over 100

Page 19 – Multiplying both under 100

Page 20 – Multiplying over/under 100

Page 21 – Double/Half Multiplication

Page 22 – Double/Double Division

Page 23 – Adding/Subtracting Fractions

Page 24 – FOIL

Page 25 – Multiplying over/under square

## 50 comments

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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

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!

Author

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

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

Author

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

u can use foil

Author

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.

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

Author

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

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.

Thanks

Author

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.

Thanks

Is there an easy way to multiply a 3 digit number by another 3 digit number

Ex:

565*344

Author

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.

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

Author

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.

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.

Author

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

Thanks. I saw it on page 28

is there a trick for multiplying 213 by 138?

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.

Author

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

you should round 138 to 140 and 213 to 200

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!

Author

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!)

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, –2) is?

Author

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

slope

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

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

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

Author

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

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

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

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

Author

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.

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

Author

Great to hear. Have a great season.

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

Author

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

This will help for math and science team, right?

I hope…

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

Author

yep its in the book and videos.

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?

Author

yep

This is really helpful math and science learned alot

Hi I like math

Author

LOL nice troll.