Wrong Simon, because the first average you give is only over 60 litres for guy one but 120 litres for guy two...
When guy one fills up another 4 times 15L making 120 litres his second lot of fill ups will attract the lower price and his 120 litres will be commensurately cheaper.
If the price goes up it's the other way round.
Sorry but my "A" level Maths is accurate this time.
Right Mr Malc, firstly there are no 2 guys in my example...read it carefully please! There is only one guy who goes to a petrol station on a Monday morning and puts in 15ltrs @ 120 = £18, the next Monday he puts in 15ltrs @ 117 = £17.55, Monday 3 he puts in 15ltrs @ 114 = £17.10, and Monday 4 he again puts in 15ltrs @ 111 = £16.65
Making a grand total of 60ltrs costing £69.30...yes?
Now just suppose on Monday One he puts in 60ltrs @ the then price of 120...he will pay £72...yes?
This fuel last until Monday 5
And then on the 5th Monday (bearing in mind he's bought the 1st 4 weeks of fuel) he buys another 60ltrs @ 111 it costs him £66.60...yes?
Now assuming you don't dispute the maths so far this means that on the 5th Monday he has bought in total 120ltrs of fuel at a cost of £72 (Monday one) and a further £66.60 (Monday two) making a total outlay of £138.60 divide this by 120 and you get 115.5 pence per ltr...yes?
Now if he'd bought it the 15ltr a week way..15ltrs @120
15ltrs @117
15ltrs@114
15ltrs@111
Equals (funnily enough) £115.5 per ltr...yes?
Now this only works if the price is falling (doesn't matter by how much or how fast) and the guy buys the same amount every time.
Malc it's standard maths....I think in your case the "A" meant Absent!