Although we had lots of answers to this problem, very few of you explained your solutions carefully. Alistair from Histon and Impington Infant School sent us this solution:
1  2  3  4  5  
1  20  30  40  50  60 
2  21  31  41  51  
3  22  32  42  52  
4  23  
5  24 

A  1  1  1  2  3  4  2  
B  1  2  3  1  1  1  2  
A+1  2  2  2  3  4  5  3  A+1 
x5  10  10  10  15  20  25  15  5A+5 
+1  11  11  11  16  21  26  16  5A+6 
x2  22  22  22  32  42  52  32  10A+12 
1  21  21  21  31  41  51  31  10A+11 
+B  22  23  24  32  42  52  33  10A+11+B 
+2  24  25  26  34  44  54  35  10A+13+B 
x2  48  50  52  68  88  108  70  20A+26+2B 
8  40  42  44  60  80  100  62  20A+18+2B 
/2  20  21  22  30  40  50  31  10A+9+B 
Chris, Isobel, Wui Shen and Alex from Maadi British International School in Cairo noticed something else:
We thought about the problem and worked it out together first. When we had our answer we tried to work out how to get our first numbers.
Wui Shen said we could take 1 away from the tens column and + 1 to the units column.
Isobel said we could + the two numbers and find two numbers that equal the same number.
We then each did our own sum and discovered Wui Shen's worked every time but Isobel's didn't because there was more than one answer to the sum.Jordan from London explained why Wui Shen's method works:
10A + B + 9 gives the final answer.