Checking if an answer is reasonable means we check to see whether an answer is a good approximation or estimate to the question. This is really important because it allows us to check whether our calculations make sense.
For example, if I said $\$10$$10 was shared between $2$2 people and they both got $\$50$$50, this would NOT be a reasonable estimate as my answer ($\$50$$50) is even more money than the total at the start ($\$10$$10)!
Well, one of the best things to do is to round the numbers in the question so you can do a quick mental calculation. If the given answer is close to your estimate, then it is a reasonable calculation.
You can also put the answer into context, ask yourself does it make sense that the answer is this big, or this small?
Is $6000$6000 a reasonable estimate for $57\times11$57×11?
Think: Let's round both these numbers to the nearest ten and see if it's close to $6000$6000.
Do: If we round $57$57 to the nearest ten, it's $60$60 and if we round $11$11 to the nearest ten, it's $10$10.
So, our estimate can be rewritten as $60\times10$60×10.
$600$600 is way less than $6000$6000 so NO $6000$6000 is not a reasonable estimate.
Ursula collects beetles. She knows that she has exactly $169$169 of them. Ursula has to move house this week, and must buy storage boxes to transport all of them.
Which of the following should Ursula buy?
Is the following calculation reasonable?
Is this statement accurate?
"$4\times88$4×88 will be less than $320$320."
Is the statement reasonable?
$9352\times4761$9352×4761 will be greater than $36000000$36000000.