mixture numbers

Question

a mixture of 5% disinfectant solution is to be made from 6% and 3% disinfectant solutions. How much of each solutions should be used if 21 gallons of 5% solutions are needed?

Answer 1

X. .06. .06x

Y. .03. .03y

21 .05 21*.05=1.05

x + y = 21

.06x + .03y = 1.05

2 equations , 2 unknowns solve with substitution or elimination!