If bob has a piece of cake that has 300 calories per serving and there are 135 calories from fat, what percentage of calories come from fat?

300 calories represents the 100% of calories on the piece of cake.

If you want to know the percentage that represents 135 calories from fat, then you have to multiply 135 calories by 100%, and then divide the outcome by 300 calories:

(135 cal * 100%) / 300cal = 45%