i do not agree with stolyar....
u are suggesting this
P(A or B)= P(A) + P(B)-P(A AND B) = 9/10
which would give u 50
this is not the case .... because it is clearly states atleast one of the two and not one of the two ....
the exp given by bat_car is correct
I offer overlapping sets.
90 is the total area, which includes A~B, B~A, and AB
at least one of the two means one, the other, or both.
So, I think I am correct