28% is the same as the decimal 0.28, which will be 28/100 as a fraction.
To simplify 28/100 you will have to find what number goes into both this will be 4.
So 28 ÷ 4 = 7
100 ÷ 4 = 25
So Elan will spend 28/100 or if you have simplified 7/25 of his workout on the treadmill.