The answer to this question is 12
Hope this help if you need help graphing I would use desmos
1000 ml costs £3.50
200 is 1/5 of 1000 so it will be £0.7
In rectangle ABCD
AB * AC = area
In another rectangle WXYZ
WX * WY = area
The sides must be equal or have same multiples and should be divisible by each other