(a) Grogg's answer was probably arrived at by considering the two cases of exactly 4 A's and exactly 2 A's separately. For exactly 4 A's, there are (46) ways to choose the 4 A's, and then 26 choices for each of the remaining two letters. For exactly 2 A's, there are (26) ways to choose the 2 A's, and then 26 choices for each of the remaining four letters. So Grogg's answer is (46)⋅262+(26)⋅264.
(b) Grogg's answer is wrong because it does not account for license plates with 5 A's or 6 A's. There are (56) ways to choose the 5 A's, and then 1 choice for the remaining letter, so there are (56)=6 license plates with 5 A's. There is only 1 license plate with 6 A's. So Grogg's answer should be (46)⋅262+(26)⋅264+6+1. The correct answer is larger than Grogg's answer because Grogg's answer does not account for all the possible license plates.
(c) The correct answer to Ms. Q's problem is (46)⋅262+(26)⋅264+6+1=15⋅676+1560+7=26353. This is the number of license plates that contain exactly 4 A's, exactly 2 A's, 5 A's, or 6 A's.
Here is a more detailed explanation of how to solve the problem:
Case 1: Exactly 4 A's. There are (46) ways to choose the 4 A's, and then 26 choices for each of the remaining two letters. So there are (46)⋅262=15⋅676=10140 license plates with exactly 4 A's.
Case 2: Exactly 2 A's. There are (26) ways to choose the 2 A's, and then 26 choices for each of the remaining four letters. So there are (26)⋅264=1560 license plates with exactly 2 A's.
Case 3: 5 A's. There are (56) ways to choose the 5 A's, and then 1 choice for the remaining letter. So there is 1 license plate with 5 A's.
Case 4: 6 A's. There is only 1 license plate with 6 A's.
The total number of license plates is 10140+1560+1+1=26353.