Question 811261
Hundreds of these examples are all the same general problem.


H = percent concentration of the higher strength material
T = target percent concentration
L = percent concentration of the lower strength material
M = size of resulting mixture
u = amount of the L strength material to use
v = amount of the H strength material to use


Your given description makes these assignments:
H = 40 %
T = 25%
L = 10 %
M = 15 liters
u and v are unknown, liters.


System of equations is:
{{{(Lu+Hv)/M=T}}} and {{{u+v=15}}}
Solve for u and v, both which are linear, so you will have two linear equations in u and v.