Question

A boy invested $860 of his Bar Mitzvah money. He put part of it in the bank at 4.5% interest and lent the remainder to his parents at 7%. If after one year the interest on these investments was $51.95, find how much did he put in the bank?

Answer 1

Let b represent the amount the boy put in the bank. Then (860-b) represents the amount he lent his parents. His earnings were ...
0.045b + 0.07(860 -b) = 51.95
-0.025b = 51.95 -0.07*860
b = -8.25/-0.025 = 330

The boy put $330 in the bank.