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.

Gasoline mixture problem

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

Considering the amount of gasoline in each one,

`100%G + 75%M = 15/16 xx 8`

That is:

`G + 0.75M = 7.5` ...(2)

We subtract equation (2) from equation (1) and get:

`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.