Is this acceptable English in Wales? If so, does it mean "subtraction problems?" It looks incorrect to me (from the US), because subtraction and sums are opposites.

I think here "sums" can be used as a general word for "doing maths".. I personally hate sums.. especially subtractions :-)

Thanks for the explanation! (BTW, in the US we do "math" and "subtraction," but I've at least heard of the phrase "doing maths" before.)