If the total number of HDDs purchased from shop A and shop B on Saturday are…

Concept: A quantity that is p% more than a base B equals B × (1 + p/100). When two different bases are each raised by their own percentage, the required total is the sum of the two separately increased quantities: a percentage acts only on the base it is attached to, so rates belonging to different bases can neither be averaged nor applied to the combined base.

Application: The Wednesday row of the table supplies the two bases.

1. Wednesday sales - Shop A: 720 HDDs; Shop B: 540 HDDs.

2. The Saturday purchase from Shop A is 20% more than 720: 720 × (1 + 20/100) = 720 × 1.20 = 864.

3. The Saturday purchase from Shop B is 30% more than 540: 540 × (1 + 30/100) = 540 × 1.30 = 702.

4. Total purchased from both shops on Saturday: 864 + 702 = 1566.

Cross-check: Work with the increments instead of the multipliers - 20% of 720 = 144 and 30% of 540 = 162, so the total increment is 144 + 162 = 306. The Wednesday combined sale is 720 + 540 = 1260, and 1260 + 306 = 1566, which matches the step-by-step total.

Contrast: Applying one single rate to the combined base is the standard trap in this pattern.

• Raising 1260 by 20% gives 1512 and raising it by 30% gives 1638; neither is admissible, because the two shops carry different rates.

• Averaging the two rates to 25% gives 1260 × 1.25 = 1575, which is also wrong: the bases are unequal (720 ≠ 540), so the effective combined rate is 306/1260, about 24.29%, pulled towards the larger base that carries the smaller rate.