If the average number of items sold by D on Monday and Tuesday is 490, then…

Concept

The average of two observations x and y is (x + y) / 2. Therefore, if the average a and one observation y are known, the other observation is x = 2a - y.

The percentage by which a new value N is more than a base value B is ((N - B) / B) × 100.

Application

1. Let D_M be D’s Monday sales. From the table, D’s Tuesday sales are 420, so (D_M + 420) / 2 = 490.

2. Thus D_M + 420 = 980, which gives D_M = 560.

3. E’s Friday sales are 800. The increase over D’s Monday sales is 800 - 560 = 240.

4. Percentage more = (240 / 560) × 100 = 42.857...%, which rounds to 42.86%.

Cross-check

The recovered value satisfies (560 + 420) / 2 = 490. Also, increasing 560 by about 42.86% gives approximately 800.

Result

Therefore, the required percentage is 42.86%.