Help! Tricky.. How do I calculate the correct oil/gasoline mixture for a two-stroke (Lambretta) engine?

Question

Or any two-stroke engine, for that matter?

All right, manual says oil and gasoline must be mixed by a 5% (or 20-to-1) ratio. That means I will have 250 mililiters (ml) of oil for each 10 liters of gasoline. Fine? Fine. But... does that mean that I will put 10 liters of gasoline, THEN add 250 ml of oil (with a total of 10.250 liters of liquids in the tank), OR do I have to REPLACE 250 ml of gasoline with the same amount of oil (9.75 liters of gasoline and 0.25 liters - or 250 ml - of oil), ending up with exactly 10 liters of liquids in the tank, but with a richer mixture? This is important in order to find the correct mixture. Thanks!!!

Answer 1

Check your math.

5% of 10 liters is 500ml.
Not 250ml.

Answer 2

Read the Instructions!!