These questions are essentially of the form percentage=number * 100/total. Your are given two parts of this equation. In this case you were given number and total and you are asked to compute the percentage. You needed to multiply number by 100 and then divide by the total to calculate the correct answer.

You have 21 out of 136 blocks. What percentage of blocks does that represent?

p=21/136
p=.154
p=15.4%