The cumulative frequency distribution shows that there are 25 runners in the age group of 20-49, and 8 runners in the age group of 20-29. Therefore, the number of runners in their forties can be calculated by subtracting the number of runners in their 20s from the number of runners in the age group of 20-49:
Number of runners in their 40s = (Total number of runners in age group 20-49 - Total number of runners in age group 20-39) =( 25 - 18) = 7
Therefore, there are 7 runners in their forties.