4. For each pair of numbers $x$, let us denote by $s(x, y)$ the smallest of the numbers $x, 1-y, y-x$. What is the largest value that the number $s(x, y)$ can take?
Answer: 1/3. Solution. The sum of the three numbers mentioned in the condition is 1. Therefore, the smallest of them is not greater than $1 / 3$: otherwise, the sum of these numbers would be greater than $1 / 3+1 / 3+1 / 3=1$. On the other hand, when $x=1 / 3$, $y=2 / 3$, the smallest of the three given numbers is $1 / 3$.