Difference in the options = $10.40 - $8.80
= $1.60

For an extra $1.60, option B gives 50mL more than option A.


For option A, since 300mL costs $8.80,
300ml ÷ 50ml = 6

$8.80 ÷ 6 = $1.4666... ≈ $1.47

Since 50ml costs $1.47 on average in option A, it is not worth to pay another $1.60 just to get another 50ml as in option B

If we are making price comparisons on a per ml basis (or per 50 ml basis or other fixed volume basis etc), then J is correct. And this is probably the answer that your teacher is looking for.

However, other factors are involved. For example, if you need exactly 350 ml of shampoo, no less, no more, then option B makes more sense than buying two large 300 ml bottles since the extra $7.20 that goes into buying the extra 250 ml unused portion gets wasted.

Alternatively, you can also make comparisons of

7 x Option A versus 6 x option B (for a common total volume of 2100 ml)

(without using a calculator, but I am sure that you can do basic primary school multiplication working on paper).