Question 478234
I used a method called allegation to work this.

Make a tic tac toe pattern

in the upper left square, put the highest percent
In the lower left square, put the lowest percent.
In the center square, put the percent you want.

 15  |   .    |   .
______

  .   |  10   |   .
______

 8   |    .   |    .

Subtract the center from the top left and place in the bottom right corner. This is how many parts you need of the 8 percent.

Subtract the lower left from the center and place in the upper right corner. This is how many parts you need of the 15 percent.


15  |   .   |  2
_____
.   |  10   |   .
_____

8   |   .    |  5

The total parts is 7.

 multiply the volume you want ( 14 ) times 2/7. This will give you the amount of 15% you will need. ( 4 liters).

Multiply the volume you want  ( 14 ) time 5/7. This will give you the amount of 8% you will need.   ( 10 liters ).

Cleomenius.