Your question is Ball and Paddle 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).
Describe a time you solved a small but non-trivial estimation puzzle, such as determining the cost of a ball when a paddle costs one euro more than the ball, and explain how you validated your answer.