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.

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.

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?

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.

## 14 comments

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## Andrew Wang

April 25, 2014 at 1:56 am (UTC 0) Link to this comment

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

Thanks,

Andrew Wang

## anthony gillespey

May 21, 2014 at 5:11 pm (UTC 0) Link to this comment

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

November 10, 2016 at 2:16 am (UTC 0) Link to this comment

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

## anthony gillespey

January 12, 2017 at 8:11 pm (UTC 0) Link to this comment

30H – 11/2 M take the absolute value.

## Math Rocks

January 27, 2017 at 3:02 am (UTC 0) Link to this comment

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

January 27, 2017 at 3:03 am (UTC 0) Link to this comment

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

## anthony gillespey

January 27, 2017 at 4:44 am (UTC 0) Link to this comment

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

January 29, 2017 at 5:52 pm (UTC 0) Link to this comment

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

## Beatriz

March 1, 2017 at 1:28 am (UTC 0) Link to this comment

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

For example: 243X514

## anthony gillespey

March 21, 2017 at 9:58 pm (UTC 0) Link to this comment

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

## NERD

February 28, 2019 at 1:59 am (UTC 0) Link to this comment

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

## anthony gillespey

May 22, 2019 at 3:46 pm (UTC 0) Link to this comment

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

## Awesome

June 14, 2019 at 5:28 pm (UTC 0) Link to this comment

How would you do 18×19/23?

## anthony gillespey

June 14, 2019 at 8:27 pm (UTC 0) Link to this comment

I have a new book coming out in days that has this trick in it.