Aboy 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?

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.