In order to solve how many problems each person has solved, you have to multiply the fraction by the number of problems on their assignment. To do this, you multiply the numberator by the number of problems and then divide that by the denominator for both Dustin and Kelsey.
For Dustin, 3/8 * 24 = 72 / 8 = 9 problems completed
For Kelsey, 4/15 * 30 = 120/15 = 8 problems completed
Dustin has completed more problems
