To start with, we made number cards using scraps of paper and arranged them in order. We then used these to make the calculations using what we know about pairs of numbers. We started by thinking of 2 numbers that add together to make 15 - we started with 5 & 10 but realised once we had make 15 that way, we couldn't go any further, so we used 6 & 9. Next we found a number to add to 9 to make 20 (11). Then we knew we needed to make 3 in the units column so used 12. We still had small numbers to use so put 4 next. After that we knew we needed to make 8 and had 14 free and finally used the 7 to make 21.
Once we had solved it the first time, we then worked with different pairs to find all the solutions.
We found 3 solutions by reversing the pairs of numbers to make 15 at the start and filled in the rest.
6, 9, 11, 12, 4, 14, 7
7, 8, 12, 11, 5, 13, 6
8, 7, 13, 10, 6, 12, 9