Mental arithmetic techniques

[SomeGuy]

DS, who is 7, has just done 25 or so sums along the lines of

149+376+458

I'm not sure if he was supposed to do them in his head or with working out, as there is no space on the worksheet for any working, but as I think faculty with mental arithmetic is useful, I've not told him to do them outright.

He got about 20/25, but had made some obvious mistakes such as '1007' as the answer to the above - to which I asked him what 9+6+8 is, to show that the answer could not end in 7.

I'm wondering what the best way to advise him to do such sums mentally is - either 100+300+400= 800, then add 40+70+50 - 160, then 9+6+8 = 23. Or just 149+376 = 525, then 525+458 = 983

He's obviously quite good at it, but I think the 20% error rate is too high to be justifiable. Just practice and let him work it out by himself? Do it twice?

I think he will settle on something for himself but otherwise I would say that the first method is easier, it's more Carol Vorderman on Countdown stylee anyway. Why not tell him both methods and he can see which he prefers?

Our school teaches the first method. My DS is 8 and they wouldn't expect it to be done without working out. They would be expected to show the working out as you have done it.

Are you sure he is meant to be finding the one correct answer or is he meant to be rounding up/down in order to estimate an approximate answer? 150 + 400 + 450

We tend to teach the first way for Y 3/4, and let them record if they want to and are unable to keep the different totals in their heads.
For a simple calculation such as 149+376, I'd also get mine to flip it round, 376+150= 426 and then subtract 1.

In your head it's easier to add up the most significant digits first ie the 100s. On paper it's the other way round.