rounding it to the nearest tens place . . . here are your options:
. . .
620
630
640
. . .
obviously 630 rounds to 630 since it is equal to it
but the rule is, if the rounded value (in this case, the value in the ones place) is equal to or greater than five (5), then you round up . . . if the rounded value is less than five (5), then you round down . . . since our value in the ones place is 0 (which is less than 5), we round down to the next multiple of ten, which happens to be 630 itself.