” The 15 sums include multiplication, subtraction, percentages and division”
If the problems to be solved are to include multiplication, subtraction, percentages and division, won’t there be products, differences, fractions and quotients as well as “sums”?
5. an arithmetical problem to be solved, or such a problem worked out and having the various steps shown.
http://www.dictionary.com/browse/sum?s=t
I had to look it up myself. This particular usage is probably used more in England than it is here.