Answer: we can make 15,504 different math tests
Step-by-step explanation:
Ok, we have a bank with 20 questions (Q1, Q2........ , Q20)
and we want to make a test with 15 of those 20 questions.
As the order of the questions does not matter in the thest ( is the same if the order is Q1, Q2, ...., Q15 than if the order is Q15, Q14, ....., Q1 )
So we only want the number of combinations.
When we have N elements and we want to create groups of K elements, the total number of combinations is:
[tex]C = \frac{N!}{(N - K)!*K!}[/tex]
Here N = 20 and K = 15
[tex]C = \frac{20!}{5!*15!} = \frac{20*19*18*17*16}{5*4*3*2*1} = 15,504[/tex]
So we can make 15,504 different math tests
