If he decides to save 10% he decided to save 38.75 since 387.50×.10 is 387500 (move decimal 4 times to figure out what the percentage of 387.50 is since the $ already has 2 numbers after the decimals and so does .10 which =38.75 this is what he saves so subtract this from 387.50 to find out what he has left to spend
387.50-38.75= 348.75