How can you use rational numbers to represent real-world problems?

First you need to know what is rational number. It is any number that can be written as fraction of 2 integers which denominator is not equal to 0.

One example of real-world problem:

Mark bought half kilograms of potatoes. If 1 kilogram costs $4 how much did he spent?

he spent 1/2*4=$2 here 1/2 is rational number.