We will let the amount of 100% gasoline be G, the amount of 75% mix be M and the final amount is 8.0 L.

The final mixture is in the ratio 15:1 which means 15/16 is gasoline.

So G + M = 8 .....(1)

and considering the amount of gasoline in each one,

We subtract (2) from (1) and have:

0.25M = 0.5

and so M = 2.

CHECK: G = 6 (since G + M = 8) and

6 + 0.75× 2 = 6 + 1.5 = 7.5 OK

So the amount of pure gasoline required is 6 L.