20% of 1000 gal = 200 gal of premium gasoline
80% of 1000 gal = 800 gal of regular gasoline
x = volume of premium gasoline that you need to add to the current mixture
60% of the new mixture is supposed to be premium gasoline. The new mixture would have a total volume of (1000 + x) gal, and in this total, you'd have (200 + x) gal of premium gasoline. So
You'd need to add 1000 gal of premium gasoline to get a mixture of 60% premium.