Question 478270
I would set up an allegation as follows:

Set up a tic tac toe board with:

40%, the highest concentration in the top left Square.
15%, the lowest concentration, in the bottom left Square.
25%, the percent we want in the middle Square.

40%   | .     .    . |  ... .
__________

  .     ..   .  |   25%  |  ..  .
__________

15%   |    ...  .     |   ... .

 In the lower right corner, put 40% minus 25%; this is the parts of the 15 % you need.
In the upper right corner, put 25% minus 15%; this is the parts of the 40 % you need.

40%   | .     .    . |    10
__________

  .     ..   .  |   25%  |  ..  .
__________

15%   |    ...  .     |   15

The total parts you need is 25.
The 40% needs 10/25, or 2/5 of 70 ml = 28 ml.
the 15% needs 15/25, or 3/5 of 70 ml = 42 ml.


Cleomenius.