Let A be the amount of 100% gasoline required. So the amount of gasoline-oil mixture will be 8.0 − A.

The amount of gasoline in the gasoline-oil mixture is

75% × (8.0 − A) = 0.75 × (8.0 − A) = 6.0 − 0.75A

A ratio of 15:1 means that out of 16 parts, 15 parts must be gasoline. So, the amount of gasoline required in the outboard engine mix is

(15/16) × 8 = 7.5

So considering the amount of gasoline in each part, we have

A + (6.0 − 0.75A) = 7.5

0.25A = 1.5

A = 6

Therefore, we need to add 6 L of pure gasoline to make the correct mix.