Answer
(a) 2200
(b) 500
Work Step by Step
(a) Draw a "cut-off" line after the hundreds place: "21". Next, look at the first number after the cut-off line: "9". If the number is 5 or above, round up to the nearest hundred: "2200".
(b) Draw a "cut-off" line after the hundreds place: "5". Next, look at the first number after the cut-off line: "0". If the number is 4 or below, round down to the nearest hundred: "500".
Note: You are rounding to the nearest hundreds, so be sure to use 0's in the tens and ones places..