If you add 5 ml to 10 gallons how many milliliters do you add to 3 gallons?

If you add 5ml to 10 gallons then this equates to adding 5/10( = 1/2) ml to 1 gallon.

Therefore, to 3 gallons you add 3 x 1/2 = 11/2 or 1.5 ml