Question

Chang and Cal both collect pearls. Chang has three more pearls in her collection than Cal. Write an expression to represent the total number of pearls in both collections.

Answer 1

Let x be the number of pearls in Chang's collection.  Let y be the number of pearls in Cal's collection.

Chang has 3 more than Cal, so x = y + 3.
Adding both collections would be x + y.  It's easier to have an expression in one variable, so we will substitute what we had written about Chang in for x.
(y + 3) + y = 2y + 3.