# Pages 26 through 46

## Page 65 -Dividing Factorials

• Andrew Wang on April 25, 2014 at 1:56 am

can you show how to do 16!/(8!*8!)

Thanks,

Andrew Wang

1. Think of it this way. (16x15x14x13x12x11x10x9x8!)/(8!x8!)= (16x15x14x13x12x11x10x9)/(8x7x6x5x4x3x2x1),cancel 8! from numerator and denominator. After that just start canceling from top and bottom. such as 16/(8×2) cancels. Keep doing that and you should end up with a single whole number answer. In this case 12870 please note that this problem could not be solved mentally for most people.

• Math Rocks on November 10, 2016 at 2:16 am

Hello Mr. Gillespy,
Can you make a video showing how to solve those clock problems? For example, on some ns test, it asks to solve for the acute angle between 5:24 or something like that. The reason I feel a video will help more than a regular explanation is because it is easier to explain visually. Any way that you can reply, I appreciate it.

Thanks

1. 30H – 11/2 M take the absolute value.

• Math Rocks on January 27, 2017 at 3:02 am

Thank you Mr. Gillespy. I have ANOTHER question. How do you divide 34 base 8 by 2 base 8 without changing each to base ten then swithcing it back. Is there a trick for it?

• Math Rocks on January 27, 2017 at 3:03 am

By the way, I forgot to say that the answer has to be in base 8

1. Honestly in this case converting to base 10 then divide by two is very fast. Then back to base 8 is another 2 seconds. That’s the way I would do it. If you wanted to though you would have to know some basic base 8 times tables or convert quickly. Example 2 goes into 3 once remainder 1 bring down 4. 3 goes into 14 base 8 (this is the tricky part) 4 time. Because 14 base 8 is = to 12 base 10. Like is said more work than its worth.

• Math Rocks on January 29, 2017 at 5:52 pm

thank you.. also thanks for the overflow calc video, it helped me

• Beatriz on March 1, 2017 at 1:28 am

Hello, do you know of a math trick to get an approximate answer when multiplying 2 and 3 digits numbers.
For example: 243X514

1. Well 243 is reall close to 250 so use the 25’s trick with 512 since its a multiple of 4. so 128000

• NERD on February 28, 2019 at 1:59 am

Mr Gillespey is it possible to show how to do overflow on a TI 84 plus CE

1. The process would be the same but honestly you should move past a ti 84.

• Awesome on June 14, 2019 at 5:28 pm

How would you do 18×19/23?

1. I have a new book coming out in days that has this trick in it.Scratch that the book is out now 🙂