Rounding and estimating
To round, look at the digit just to the right of where you are rounding: 5 or more rounds up, 4 or less rounds down. 3,746 to the nearest hundred is 3,700; 3,756 is 3,800.
Estimate before you calculate. 49 × 21 is about 50 × 20 = 1,000, so an answer of 102.9 or 10,290 is clearly wrong. On a multiple-choice test, an estimate often picks the answer by itself.
AFOQT questions on this: Comparing values · Large numbers