Question 969618
Let x= number of liters of 6% sulfuric acid solution.

Let y= number of liters of 15% sulfuric acid solution.

x+y=10  (units are liters)

.06*x + (.15)y=(.09)*10

y=10-x  (substitution works well here).  Elimination could be used.

.06x + (.15) (10-x)=0.9  (I multiplied out the right side)

.06x + 1.5 - 0.15x=0.9

Subtract 1.5 from both sides and combine x s    You will get two negatives, one on each side of the equation, and they will cancel out, making the result positive.

-0.09x=-0.6
x=6.67 (repeating decimal) liters of 6%
y=3.33  liters of 15%

Check is done a different way:
9%, the desired amount, is 1/3 the way between 6 and 15%

Therefore, 2/3 of the solution must come from the 6% solution and 1/3 from the 15% solution