Let a and b be subsets of a universal set u and suppose n(u) = 400, n(a) = 115, n(b) = 90, and n(a ∩ b) = 50. find the number of elements in the set.

By the inclusion/exclusion principle,

There are 400 elements in the universal set , which means there are 400 - 155 = 245 elements not accounted for by , or

That's all you can really determine from the given info. Considering the language of the problem, "Find the number of elements in the set", I find it hard to believe that the set it's talking about isn't mentioned.