if the ave. gas consumption for 12 months is £52/month, how much did it cost 3 people for 5 months and 4 for 7

Question

£52 as an average does not take into account what 3 people paid for 5 months and when another person joined the house what 4 people should have paid for the remaining 7 months.
Assuming the useage is the same per person, what would it have cost for these amounts of people over these periods.

Answer 1

3 for 5 m =£86.66 each,
4 for 7m =£91.00 each for the 7 m

Answer 2

It would cost each person about £14.51 per month.

Proof

3 people x 5 months = 3 x 5 x £14.51 = £217.65
4 people x 7 months = 4 x 7 x £14.51 = £406.28

Total £623.93

12 months at £52 = £624