A delivery rider has been on time for 42 of his first 60 runs, and 20 runs are still to be made. At most how many of those 20 runs may be late if he is to be on time for at least 75% of all 80 runs?

He must be on time for at least 75% of 80 = 60 runs, so he may be late for at most 80 - 60 = 20 runs in all. He has already been late for 60 - 42 = 18, leaving room for just 2 more. 5 is 25% of the 20 runs left, applying the allowance to the remaining runs only and ignoring the 18 lapses already on the record. 20 is the whole lateness allowance, forgetting that most of it is already used. 18 is the number of the last 20 runs he must be ON TIME for, quoted in place of the ones he may miss.