A biologist has two brine solutions, one containing 7% salt and another containing 28% salt. How many milliliters of each solution should she mix to obtain 1 L of a solution that contains 19.6% salt?

Each litters can be represented as; x and y respectively to the 7% and the 28%

x + y =1

7x + 28y =19.6

This a simultaneous equation and can either be solved by using elimination or substitution method

Using the substitution method

eqn1, x=1-y

putting into eqn2

7(1-y)+28y =19.6

7-7y+28y=19.6

21y=12.6

y=3/5

but x=1-y

so

x=1-3/5

x=2/5

2(2x + 3) = -4 + 2(2x + 5)

4x + 6 = -4 + 4x + 10

4x + 6 = 4x + 6

4x - 6 + 6 = 4x - 6 + 6

4x = 4x

x = x

unlimited solutions.