The school library has 60% more books than the club library. The club library has how many per cent fewer books than the school library?

Let the club library hold 100 books; then the school library holds 160. The shortfall of 60 books is now measured against 160: 60/160 = 0.375 = 37.5%. 60% is the trap of assuming the comparison is symmetric - it is not, because the base changes from 100 to 160. 62.5% is 100/160, the club total as a share of the school total rather than the gap, and 40% is a careless 100 - 60.