Vinegar and math!

Question

How much 90 percent vinegar do you have to add to a gallon of 5 percent vinegar to get 20 percent vinegar?

Answer 1

 

   0.9 x + 0.05 = 0.2(x+1)

 

 9x + 0.5 = 2(x+1)

 

 90x + 5 = 20(x+1)

 

 90x + 5 = 20x + 20

 

 70x +5 = 20

 

70x = 15

 

x = 15/70 = 3/14