Your question is Two-Weighing Coin Puzzle. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You have eight identical-looking coins, but one is counterfeit and weighs slightly less than the others. Using a balance scale, how can you find the counterfeit coin in exactly two weighings?