On most machines, you enter quantity X amount like you have shown, e.g type 1.75 then press X which is usually near the decimal point button (USA model is labelled a/FOR) then type the price (NO DECIMAL POINT) so 312 for 3.12 per gallon then press a department button (one of the item buttons usually to the right of the numeric pad)
Note: some machine do not allow 2 decimal places, so you might have to use 1.7
See how you get on and let me know if you get stuck
Mark, January 2010