Answer:
4 liters for £3.20
Explanation:
we want to maximize quantity for each unit of money spent. if we use mL and p, we want to maximize mL/p
500 mL = 60 p
divide both sides by 60p to get a mL/p value
500 mL/60 p = 50/6 p ≈ 8.3 mL per pence
4 L = £3.20
note:
1000 mL = 1L
divide both sides by 1 L/ 1000 mL
1000 mL / 1 L = 1
1 L / 1000 mL = 1
100 p = £1
divide both sides by 100p / £1
100p / £1 = 1 = £1 / 100p
convert everything to the same units (mL and p) to accurately compare
4 L = £3.20
4L = 4L * 1 = 4L * 1000 mL / 1 L = 4000 mL. Note that we have two options to choose from -- 1000 mL / 1 L and 1 L / 1000 mL. We choose the first one here because we want the 4L to cancel out with the 1 L
£3.20 = £3.20 * 1 = £3.20 * 100p / £1 = 320 p. note we put the £1 on the bottom so the euros cancel out.
thus,
4 L = £3.20
= 4000 mL = 320p
divide both sides by 320p to get a mL / p value
4000 mL / 320 p = 12.5 mL/p
8.3 mL per pence in the first one
12.5 mL per pence in the second one
the second one has a higher mL per pence, so that is a better value