# Petrol Price Math

For the last couple of weeks I’ve filled up at the same petrol station, and then entered the details into Gas Cubby and the docket price is 2 cents different from that displayed on the ipod….

2.119 x 27.94 = 59.20486, which should round to 55.20

I could forgive 55.21, but really 50.22, that’s magic rounding, or they are using more decimal places than shown.

To make the checking easy I’ll assume infinite precision and thus the rounding will trend towards .5 of the last digit. 2.1195 x 27.9405 = 59.2198898 thus 59.22, so it could be both figures are misrepresented or just the \$/litre, as when you don’t fudge the price and only the litre amount the total rounds to 59.21.

Or maybe they  just have bad programmers and the company is not evil.

Which reminds me of how my TelstraClear (now Vodaphone) bill never added up. If you telly the sub totals reported to 2 decimal places, the total was off 1 to 2 cents every bill. Scary was that two of three sections were fixed price, and the third was fractional. So how they did that I was never sure.