Estimating


Remember, when we estimate we are making a guess at the answer – not a wild guess but an educated guess.


A good estimate is an estimate that is close to the real answer.


The children in St Vincent’s primary school collected €5478 for charity. The children in Scoil Caitriona collected €7621 for the same charity. Estimate the difference between the two sums of money collected. Discuss your estimate with your partner. Did you make a ‘wild guess’ or an ‘ educated guess’?


Remember: When using bigger numbers we round to the nearest 100. Example: €7621 – €5478 = ?


€7621 rounds down to €7600. €5478 rounds up to €5500.


We know that €7600 – €5500 = €2100. Now let’s work out the answer properly to see how good our estimate was!


Th H T U 7 6 2 1


5 11 1


– 5 4 7 8 2 1 4 3


Our estimate – €2100 – is close to €2143, so it is a good estimate.


Now try these. Estimate the answer first and then work out the exact answer on your calculator.


1. a) 8751 – 3214 = Estimate c) 3888 – 1238 = Estimate e) 5204 – 4686 = Estimate g) 6202 – 1857 = Answer h) 1635 – 1289 = Subtraction 25 Answer f) 7364 – 5555 = Estimate Answer Answer d) 4267 – 1056 = Estimate Answer Estimate Answer b) 6325 – 2214 = Estimate Answer Estimate Answer


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