Maths SAT Paper A 2010: Questions 17 and 18

Question 17 of the Maths SAT Paper A 2010 asks for three different numbers that add up to 40. Each number is less than 20 and each number is even.Surprisingly there are only four possible answers:
18 + 16 + 6
18 + 14 + 8
18 + 12 + 10
16 + 14 + 10
One mark given for any of these. The numbers can be given in any order.
Suggested method:
There are two key points here: firstly each even number is below 20 and secondly all the numbers are different.
I approached this by thinking what was the first even number below 20, which of course is 18. I took 18 from 40, leaving 22. I then thought of two even numbers that made 22. (20 and 18 had already gone) so 16 and 6 would be fine.

Question 18 is all about negative numbers and counting back in steps of 125.

Subtracting 125 from 50 is tricky and best done making notes rather than trying to write a standard subtraction method out. The best way is to subtract 50 taking the number to zero. Work out that 75 more needs to be subtracted, making -75.
Subtracting another 125 is easier now that both numbers are negative. Mentally adding 75 and 125 will give 200 so the same process with negative numbers will give -200.
Children find negative numbers quite tricky and this is a question which many will get wrong.

Questions 17 and 18 from SATs Paper A 2010

Questions 17 and 18 answers and suggested methods.

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