Estimating to check your answers

Mathematics · Multiplication & Division · ages 10-11

Name ______________________ Date ____________

Round 487 to the nearest hundred.
Answer: ______________
Which is the best estimate for 487 x 23?
- 500 x 20 = 10,000
- 400 x 20 = 8,000
- 487 x 23 = 11,000
- 500 x 30 = 15,000
To estimate 487 x 23, which rounded numbers would you multiply?
- 500 x 20
- 480 x 25
- 500 x 25
You're paying a bus fare and guessing how many sweets fit in a jar. Which one needs an exact number?
- The bus fare
- The sweets in the jar
- Both need only an estimate
Estimate 52 x 48 by rounding both to the nearest ten.
Answer: ______________
A builder gives a rough quote for how many bricks a wall needs. Later, they order the bricks for delivery. Which stage needs the exact number?
- The rough quote
- Ordering the bricks for delivery
- Neither stage needs an exact number
A pupil computes 487 x 23 = 1,201. Using 500 x 20 = 10,000, what can you say?
- The answer is correct
- The answer is far too small
- The answer is slightly big
- Estimates prove nothing
Rob says 51 x 19 is about 1,000 because 50 x 20 = 1,000. Is Rob right?
Circle one: True False
Tom estimates 293 x 41 as 300 x 40 = 12,000. He then works out the exact answer and gets 1,201. What does this tell Tom?
- 1,201 is correct, since it's close to 293
- He has probably made a mistake, since 1,201 is far smaller than his estimate
- His estimate of 12,000 must be wrong
Which answer is reasonable for 203 x 49?
- 994
- 99,470
- 104,947
- 9,947

Answer key

For grown-ups. Fold this page away before handing over the rest.

500 · The answer is 500 because 487 sits between 400 and 500, closer to 500.
500 x 20 = 10,000 · 500 x 20 = 10,000 is best: 487 rounds to 500 and 23 rounds to 20.
500 x 20 · 487 rounds to 500 and 23 rounds to 20, so the estimate is 500 x 20.
The bus fare · Money needs to be exact so you pay the right amount. Guessing sweets in a jar is fine as a rough estimate.
2500 · The estimate is 2,500 because 50 times 50 equals 2,500.
Ordering the bricks for delivery · A quote can use a rounded estimate, but an order needs the exact number so nothing is wasted or missing.
The answer is far too small · The answer is far too small: the true value is near 10,000, but 1,201 is about ten times too small.
True · Rob is right. His roundings are honest and 50 times 20 is exactly 1,000.
He has probably made a mistake, since 1,201 is far smaller than his estimate · 1,201 is about ten times smaller than the estimate of 12,000, a sign a digit went missing somewhere in the calculation.
9,947 · 9,947 is reasonable: the estimate 200 x 50 = 10,000 hugs it, and 203 x 49 = 9,947 exactly.