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how much 15% antifreeze should be mixed to get 50 gallons of 30% antifreeze 

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With what are you mixing the 15% antifreeze? 

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Andy C. | Math/Physics TutorMath/Physics Tutor
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There is one chemical on hand that contains 15% antifreeze. Call this chemical A.
Let k be the percent of anti-freeze in the other chemical that is NOT given here. Call this chemical B.
Note that k is not 15%, for otherwise it would be the same as chemical A.
 
Let A and B represent the amounts of each chemical needed.
 
A+B = 50 ---> B = 50-A
 
The amount of anti-freeze is  0.15A + kB = 0.3*50 = 15
 
0.15A + k(50-A) = 15
 
0.15A + 50k - Ak = 15
 
0.15A - Ak = 15 - 50k
 
(0.15-k)A = 15 - 50k
 
A = (15 - 50k)/(0.15 - k)  
 
B = 50 - A  = 50 - (15 - 50k)/(0.15-k)
 
 
                   = [50(0.15 - k) - (15 - 50k) ]/(0.15 - k) 
 
                    =  [ 7.5 - 50k - 15 + 50k]/(0.15 - k)
 
                     = (-7.5)/(0.15 - k)
 
So in terms of k,
A = (15 - 50k)/(0.15 - k) and B = (-7.5)/(0.15 - k)
 
 
Now we test it.
Suppose that chemical B contains k=50%=0.5 anti-freeze
 
Then A = (15 - 50*0.5)/(0.15 - 0.5) = (15 - 25)/(0.15-0.5) = (-10)/(-0.35) = 28 and 4/7
        B = (-7.5)/(0.15 - 0.5) = (-7.5)/(-0.35) = 21 and 3/7
 
Note that A+B=50 and
 (0.15)(28 and 4/7) + (0.5)(21 and 3/7) = 4 and 2/7 + 10 and 5/7 = 15 = 50*0.3 = 30% of 50
 
So yes, the formulas work.
 
So WHEN you find out what percent of anti-freeze, k,  is in the second chemical (chemical B)
you plug it into the formulas I have derived for in bold above to get your answer.
 
We have given this problem it's due diligence, and it is the best we can do for you.
Good luck.