Question 551305
Let
A= the amount of 25% kool aid
B= the amount of 40% kool aid

2 equations are needed

1.) A+B = 50 gallons of kool aid
2.) .25(A) + .40(B) = .30(50)

1.)rewrite 1st eqaution as such... B = 50-A

2.)substitute newly written equation as such... 
       .25(A) + .40(50-A) = 15
       .25(A) - .40(A) + 20 = 15
       -.15A = -5 
           A = (approx) 33.3 gallons
3.) use 1st equation to solve for B
    33.3 + B = 50
    B = 16.7 gallons

4.) Final Answer 
    A = 33.3 gallons, B = 16.7 gallons

hope this helps :)