Blocked Off

Jeff had fewer than 100 blocks. When he made five equal rows, he had one left over; with four equal rows, he had one left over; and with nine equal rows, he had none left over. How many blocks did Jeff have?

Extension:
Suppose that all the conditions about Jeff’s blocks remain except that he has fewer than 1000 blocks. How many blocks might Jeff have?

(Source: Adapted from Mathematics Teaching in the Middle School, Jan-Feb 1995).