Linear equations two variables

Question

I need 55% alcohol solution. I have 120 mL of 45% alcohol mixture. You also have 95% alcohol mixture. How much of 95% mixture will you need to add to obtain the desired solution ?

Answer 1

120 * 0.45 + 0.95 * x = (x+120)* 0.55

120 * 45 + 95x = 55(x+120)

5400 + 95x = 55x + 6600

5400 + 40x = 6600

40x = 1200

x= 30