A table of numbers:
49, 45, 42
55, 51, 59
54, 48, 52
When you select each one, you find out if it rounds to 50.
45, 48, 49 all round up to 50.
51, 52, 54 all round down to 50.
Three numbers do not round to 50.
42 rounds down to 40.
55 and 59 both round up to 60.