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Here is our answer to the Nrich number problem:

We think that the digits in the numbers above ten will always add up together to equal the numbers in order up to 10: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
E.g. 19 = 10 20 = 2 21 = 3 22 = 4 etc. up to 30
This means that when you subtract that number, they will all equal the same thing. The reason behind this is that each time, both numbers will get bigger by one.
Using the example earlier:
E.g. 20 - 2 = 18 21 - 3 = 18 22 - 4 = 18 etc. up to 30

We now have the example ‘25’. 25 is seven away from the last multiple of 9. Then, when you add 2 and 5, it equals 7, the distance it is away from the last multiple of 9.