Mary needs to read 3 books for her english class. her teacher has given her a list of 7 books to choose from. how many different ways can she select the books? 45 35 65 55

Choosing "r" things from a set of "n" can be done in this many ways:
combinations = n! / [r! * (n-r)!]
combinations = 7! / [3! * 4!]

combinations = 7 * 6 * 5 / 6
combinations = 35

Source:http://www.1728.org/combinat.htm

Hello!

the answer is:

the answer is:

why?

to solve the problem, we need to remember the quotient of power property, it's defined by the following relation:

if we have a quotienf of powers that have the same base, we need to keep the same base and subtract the exponent of the denominator power to the exponent of the numerator power.

so, we are given the expression:

then, calculating we have:

hence, the answer is:

have a nice day!

So confused right here i need it to be more clearer