Ms LFC123, it's a famous puzzle in math, in case you don't know! and albeit it can be solved by boolean logic...the most accomplished method that i know and the world has known as of now, is first order logic. i would have typed the whole solution mathematically but i am too busy for it right now! if you want us to reiterate the answer or applause you for raising a great question of all time in sajha, here's the answer for you for your solace both are knave g'night LooTe