How to see it
Four quick tools. Estimating: round, then work, then compare. Number bonds: pairs that make 10, 100 and 1,000. Patterns: spot the repeat before you calculate. Friendly numbers: turn a hard sum into an easy one.
A worked example
49 times 21 is about 50 times 20, 1,000. That tells you the answer is close to 1,000. Then if the options are 1,029, 980, 1,290, 1,092, only one is close and has the right last digit: 9 times 1. 1,029.
The common mistake: setting out a long method for a quick sum
The trap is a full column method for everything. It is slow, and more steps means more slips. Estimate first, and use the options to your advantage.
Try one
About how much is 98 plus 205 plus 97?
- A300
- B400
- C500
- D600
Show the answerHide the answer
Right: B, 400
100 plus 200 plus 100, and the small differences cancel out.