I assume A ∆ B denotes the symmetric difference of A and B, i.e.
A ∆ B = (B - A) U (A - B)
where - denotes the set difference or relative complement, e.g.
B - A = {b ∈ B : b ∉ A}
It can be established that
A ∆ B = (A U B) - (A ∩ B)
so that
n(A ∆ B) = n(A U B) - n(A ∩ B) = 10 - 3 = 7
Not sure what you mean by p(A ∆ B), though... Probability?
