Achemist currently has two solutions of sodium chloride.
one solution has a 5% concentration and the other has a 25% concentration.
the chemist needs to make 10 l of a 10% sodium chloride solution.

The amounts are x and 10-x

so

0.05x+0.25(10-x)=0.1(10)
0.05x+2.5-0.25x=1
2.5-0.2x=1
minus 2.5 both sides
-0.2x=-1.5
divide both sides by -0.2
x=7.5
sub back

other one is 10-x=10-7.5=2.5

7.5L of the 5%
3.5L of the 25%