HELP!!!!!!15 students, 12 in each photo. How many different combination is that?

The problem is a basic n choose k problem. Here's how to solve one of those:

The formula for these problems is C(n,k) = n!/k!(n - k)!. In this problem, n=15 and k is 12. Substitute the variables.

C(15,12) = 15! / 12! (15 - 12)! Now solve. ! means factorial and is done like this n! = n*(n-1)*(n-2)...*1, so 15!= 15x14x13...x1=1,307,674,368,000

C(15,12)= 1,307,674,368,000 / 479,001,600*3!= 1,307,674,368,000 / 479,001,600*6 = 1,307,674,368,000 / 2,874,009,600

Dividing the two numbers will get you 455.

There is a function on most scientific and graphing calculators that will do all of this for you. It usually is nCr, but if you need to show all of the steps, there is usually a factorial function, too.

U do 15 times 12 times the number of photos

You can omit 3 students:

(15 choices for 1st student to omit) x (14 choices for 2nd student to omit) x (13 choices for 3rd student to omit)

But these 3 can be arranged in 3x2x1 ways.

So the answer is (15 x 14 x 13) / (3 x 2 x 1)

which is sometimes abbreviated 15C3 or:

(15)

(3)

If you work out 15C12 you get the same answer.

you can multiply 15*12 and it'll equal 180

15c12

15! / [12!(15 - 12)!]

455

15x12= your answer.