Ok first We need to have the facts. He has 120 in total and 54 were damaged by the flood. So he has 56 remaining that weren't damaged. That really won't matter here.
What we are asked here is what PERCENT/% of the comics were damaged and we know 54 of 120 were damaged, 54/120.
To find the percent we divide the part damaged (54) by the whole (120) and then multiply it by 100x so 54÷120 = .45
then .45 x 100 = 45 which is the same as 45%
*45% of the comic books were damaged* ;)