Osborne said he made it a "rule in life not to answer a load of maths questions". So he refused to answer. (It's 56, since you asked.)
Riddings was one of a number of children appearing on Sky News as part of a question and answer session organised by the newspaper First News. The panel of schoolchildren also got Osborne to say he likes listening to Pharrell Williams while running around the park. So that's that, then.
Watch the full interview here.