I think the question is pretty clear. Part a) talks about numbers n such that 4 divides n or 5 divides n or 20 divides n, i.e., any of the numbers 4, 5, 20 (or possible two or all three of them) divide n. Part b) talks about n such that there is exactly one number k in {4, 5, 20} such that k divides n.