Did you mean that 20% of the people in the 5 km race are in 35-39, instead of 3 - 39? Because if so, all you have to do is multiply 360 ( the number of the total runners) by 0.20 (the percentage number of the people in the 35 - 39 age group) and the answer is the number of people. The answer is 72 people are in in the 35 - 39 age group.
