Most of the information's are already given in the question. It is important to understand the language of the problem for solving the problem. Once the language is understood, then solving the problem is easy.
Let us assume the number of black jelly beans = x
Number of yellow jelly beans = y
Then from the question we can get to a equation.
3x - 7 = y
We also know from the question that
x + y = 225
x + 3x -7 = 225
4x = 225 + 7
4x = 232
x = 232/4
= 58
So Cassidy has 58 black jelly beans out of the total of 225 jelly beans
