Quick maths tricks

[firebrand1]

Has anyone ever seen or collated a set of quick maths tricks, to speed up mental maths eg

1. x5 is the same as x10 and halve. So 18x5 is 18x10 = 180, then halve =90.

2. x8 is the same as double, double, double. So 36x8 foes 72, 144, 288.

That kind of stuff.

[neurotic kent mum]

This is very helpful - any other tips?

[Guest55]

These are both routinely taught at Primary school ...

[Okanagan]

A number is divisible by three if adding all the individual digits together gives a number which is divisible by 3. Repeat until you get down to a number you recognise from 3 times table.

[doodles]

My eldest DS was taught BOGOF's (buy one get one free) for his tables i.e. learn the "easier" table and the harder one comes free if you reverse it.

i.e. 5 x 7 = 35 / 7 x 5 = 35

4 x 8 = 32 / 8 x 4 = 32

[bravado]

My DD, who is going in to YR6, didn't know about Firebrand's incredibly useful tips - perhaps she'll be taught it in YR6..or should this have been routinely taught before YR6?

[Guest55]

All these should be taught before Year 6 -

[bravado]

Guest55 - Hmmm - maybe Essex schools are different to your area?
If it's a case that these things are supposed to be routinely taught, then that's a little worrying, especially as my DD is in a school where they're proud to say that they cream off the most able children in Maths and English and set them higher challenges in extension classes. DD achieved Level 5's across the board at the end of Yr 5 - so I would have hoped she'd have been taught these, if they're routinely taught at KS2

[Okanagan]

Is a number divisible by 7? Multiply the last digit by 2 and subtract the answer from the remaining digits. If this number is divisible by 7 then the original number is too. Like the divisible by 3 trick this one can be repeated until the number is recognisable from times tables.

e.g. 644 - multiply the last digit (4) by 2 to give 8. Subtract 8 from the remaining digits (64) to give 56, which is of course divisible by 7, so 644 is too.

bigger number 4137 - multiply the last digit (7) by 2 to give 14. Subtract 14 from the remaining digits (413) to give 399. Repeat the process by multiply the last digit (9) by 2 to give 18. Subtract 18 from the remaining digits (39) to give 21, which is of course divisible by 7.

[neurotic kent mum]

Any more - my dc certainly not taught these at ks 2. And nor was I so really grateful for this advice.