Explanation:
12 bottles for 0.90 per bottle = 12×0.90 = £10.80 as initial cost.
1/4 of 12 = 12×1/4 = 12/4 = 3 bottles
so, 3 bottles are sold for £2.10 per bottle = 3×2.1 = £6.30
the rest (12-3) 9 bottles are sold for £2.40 per bottle = 9×2.4 = £21.60
so, the total sales was 6.3 + 21.6 = £27.90.
the profit is then sales minus initial cost :
27.9 - 10.8 = £17.10
1% of the initial cost is 10.8/100 = 0.108
how often does 1% fit into the total profit ?
17.1/(10.8/100) = 17.1×100/10.8 = 158.3333333...%
so, the profit is 158.3333333...% of the initial cost.