In a mathematics test, the average marks of sections A, B and C are 85, 75 and 83 respectively. If the average marks of sections A and B are 79, and the average marks of sections B and C are 80, then what were the average marks of the three sections?

Correct option is C

Given:
Average marks of section A = 85
Average marks of section B = 75
Average marks of section C = 83
Average marks of sections A and B = 79
Average marks of sections B and C = 80

Formula used:

$$\text{Average} = \frac{\text{Sum of all observations}}{\text{Number of observations}}$$

Solution:

Average marks of A, B, and C: 85, 75, 83 respectively.

Total average mark of  A + B + C = $$\frac{85 + 75 + 83}{3}$$

Total average mark of  A + B + C = $$\frac{243}{3}$$

Total average mark of  A + B + C = 81

So, 81 was the average marks of the three sections.

Thus, the correct answer is (c).