What number should be added of the four numbers 8 21 13 31 so that they become proportional?

To make the four numbers 8, 21, 13, and 31 proportional, we need to ensure that the ratiOS of the pairs are equal. Let the number to be added be ( x ). The condition for proportionality can be expressed as ( \frac{8}{21} = \frac{13}{31 + x} ). Solving this equation for ( x ) gives ( x = 3 ). Thus, adding 3 to 31 makes the numbers proportional.