Problem 3.6. Petya can draw only 4 things: the sun, a ball, a tomato, and a banana. But he does it extremely realistically! Today he drew several things, among which there were exactly 15 yellow, 18 round, and 13 edible. What is the maximum number of balls he could have drawn?

Petya believes that all tomatoes are round and red, all balls are round and can be of any color, and all bananas are yellow and not round.
Answer: 18.

Solution. Since there are a total of 18 round objects, and all balls are round, there were no more than 18 balls. Note that there could have been exactly 18, if 2 yellow balls, 16 green balls, and 13 bananas were drawn.