Class A and class B took the same test.

[Vincen]

Class A and class B took the same test. The average (arithmetic mean) score for class A was 79.5. What was the average score for all the students in both classes?

(1) The average score for class B was 80.5.
(2) Class B had 25 fewer students than class A.

The OA is E.

Can someone help me here? Thanks.

[GMATGuruNY]

Class A and class B took the same test. The average (arithmetic mean) score for class A was 79.5. What was the average score for all the students in both classes?

(1) The average score for class B was 80.5.
(2) Class B had 25 fewer students than class A.

Each statement is clearly insufficient on its own.

Statements combined:
To make the math easier, test smaller values for the two averages.
Let the average score in Class A = 1 and the average score in Class B = 2.

Case 1: 30 students in Class A with an average score of 1, 5 students in class B with an average score of 2
Average for all 35 students = (30*1 + 5*2)/35 = 40/35 = 8/7.

Case 2: 35 students in Class A with an average score of 1, 10 students in class B with an average score of 2
Average for all 45 students = (35*1 + 10*2)/45 = 55/45 = 11/9.

Since the average for the two classes can be different values, INSUFFICIENT.

The correct answer is E.