If the average number of Printers and Speakers sold by shop C is 350, then…

Concept

For two quantities x and y, their average is (x + y) / 2.

A stated fraction of a quantity can be reversed by dividing the known value by that fraction.

Application

1. Let P be the number of printers sold by shop C. Since half of P is 300, P / 2 = 300, so P = 600.

2. Let S be the number of speakers sold. The given average is 350, so (P + S) / 2 = 350. Therefore P + S = 700 and S = 700 - 600 = 100.

3. Let C be the number of cameras sold. Cameras are four times speakers, so C = 4 × S = 4 × 100 = 400.

4. The required average is (C + P) / 2 = (400 + 600) / 2 = 1000 / 2 = 500.

Cross-check

(600 + 100) / 2 = 350 and 400 = 4 × 100, so both given conditions are satisfied.

Result: The average number of Cameras and Printers sold by shop C is 500.