Final answer:
Gary had 7 rebounds and Mark had 21 rebounds.
Step-by-step explanation:
To determine how many rebounds Mark had, let's assume that Gary had x rebounds. According to the information given, Mark had three times as many rebounds as Gary. So, Mark had 3x rebounds. The total number of rebounds they had together is 28, so we can write the equation: x + 3x = 28. Combining like terms, we find 4x = 28. Dividing sides by 4, we have x = 7. Therefore, Gary had 7 rebounds and Mark had 3 times that, which is 21 rebounds. The method used to solve for x by setting up and solving the equation x + 3x = 28 is an application of algebraic reasoning to find the individual rebound counts for Gary and Mark.