Cost price of 84 notebooks is equal to the selling price of 60 notebooks. The…

Concept: When a problem states that the cost price of one quantity of items equals the selling price of a different quantity of the same item, the gain or loss percentage is still found using the standard formula: Gain% = [(SP - CP)/CP] x 100, applied to one convenient batch (here, the 60 notebooks actually sold).

Application:

1. Let the cost price of 1 notebook = x. Then the cost price of 60 notebooks = 60x.

2. The cost price of 84 notebooks equals the selling price of 60 notebooks, so the selling price of 60 notebooks = 84x.

3. Gain on the 60 notebooks = SP - CP = 84x - 60x = 24x.

4. Gain% = (Gain / CP) x 100 = (24x / 60x) x 100 = 40%.

Cross-check: Shortcut check: whenever the cost price of m units equals the selling price of n units (with m > n), Gain% = [(m - n)/n] x 100 = [(84 - 60)/60] x 100 = 40%, which matches the step-by-step result.

Result: Hence, there is a gain of 40%.