To increase anamount by 24%, what single multiplier would you use?

24% equals .24, so you are adding .24 of the number to that number. You must also take into account the original number which counts for 100%, or just 1. Because the number is being increased, add the 24%, or 0.24, to 100%, or 1.    1 + 0.24 = 1.24. Then use 1.24 as a multiplier to increase the amount by 24%.

$40/4 fees
40/4=10
1 late fee=$10