Dt1 used to hate doing this. He'd say why would he want to guess and get it wrong when he could just work it out and get it right. I used to try and get him to estimate by rounding up or down ie to the nearest 5. But I think he still used to 'cheat' and work it out.
Both DDs find estimation tricky. Certainly DD2 agrees with Meita's DT1 - why estimate if you can work out the answer.
With DD2 I've tried to get her to appreciate that the example is an easy problem - but that the idea of estimation is useful in situations where you're not totally sure you've got the correct answer or don't quite have all the information.....
So in 'real life' I know that movie tickets are about £5 each - so if all 4 of us go I'll need at least £20 (I'm estimating how much to bring/ spend).
I do this a lot in real life with things on sale for £4.99/ £19.99 etc... rounding up £4.99 to £5 and £19.99 to £20 - to estimate it will cost me about £25 in total.
when you have figures - so say estimate the answer 5 x 8 - there will always be the kid that says 40 (DD2 would be one) - but what they're looking for is for you to work with 'easier numbers' - so 8 is pretty close to 10 - and 5 x 10 = 50. Therefore you can estimate that the answer if 50 and know that since you rounded 8 up to 10 the answer will be a bit less than 50.