If Oliver made 10% more than he usually makes, he would not have $520 in his savings.
520 × 10% = 52
520 + 52 = 572
10% × 572 = 57.20
572 - 57.20 = 514.80
Oliver would have $57.20 saved. So, he would have $514.80 left to spend. Instead of adding 10% to his usual earnings, Oliver multiplied his earning by 10, falsely concluding that his spending money would be higher than what it actually is.