We first must calculate how many ways 2 oblects can be chosen from 5.
combinations = 5! / 2! * (5-2)!
combinations = 5*4 / 2
combinations = 10
There are 10 ways to choose the 2 buttons and 5 ways to choose the final butto so there are 10 * 5 = 50 different ways.
Source
1728.com/combinat.htm
The answer would be d , because you have to add how many of each fish to equal up to the sum of 420 . which is x y . so the first equation is x + y = 420 . then it says he bought 6 times as many parrotfish as he did angelfish so you would have to multiply x which represents angelfish by 6 . to get the second equation of y = 6x .
To put this into an equation, we can make x the amount of two year old's there are.
x+2x+2x=45
5x=45
x=9
9 two year old's, 18 three year old's, and 18 four year old's.