percentages of gallons and disinfection

Question

A mixture of  disinfectant solution is to be made from  and  disinfectant solutions. How much of each solution should be used if  gallons of the  solution are needed?

There needs to be ????? gallons of  solution and ?????? gallons of solution.

Answer 1

7/3 = x/y where x+y = 30

y = 30-x

7/3 = x/(30-x)

210 -7x = 3x

10x = 210

x = 21 of 20%

y = 9 of 10%

,2(21)+,1(9)

= ,4,2 +.9

= 5,1'

= ,17(30)= 5.1