Question: there are eight balls seven weigh the same?
Answer: One of them is heavier than the rest of the 7 (all the others weigh exactly the same). You a provided with a simple mechanical balance and you are restricted to only 2 uses. ... If they balance out, then 3 is the heavier one. If the right side is heavier, then we know that either 4, 5 or 6 is the heavier ball.