Solution for #24

First figure out how much three of each time of stamp costs (63 cents). Now you need to figure out which two stamps to get a fourth of that will make the total cost some number of cents divisible by ten. You can't get enough to make 80 cents with two stamps, so the total cost must be 70 cents and the cost of the two extra stamps must be 7 cents. So the two extra stamps are a two-cent stamp and a five-cent stamp.