Answer: a) Each group has 5 members as its greatest possible.
b) There are 4 groups of sixth graders and 5 groups of seventh graders.
Explanation:
Since we have given that
Number of sixth grader = 20
Number of seventh grader = 25
We need to find the greatest possible number of members in each group will contain:
For greatest possible number we will find "Highest common factor (H. C.F) of 20 and 25 = 5
a) Each group has 5 members as its greatest possible.
b) Number of groups of sixth graders would be

Number of groups of seventh graders would be

Hence, there are 4 groups of sixth graders and 5 groups of seventh graders.