i need help i dont get this problem

Question

You need a 70% alcohol solution. On hand, you have a 90 mL of a 5% alcohol mixture. You also have 85% alcohol mixture. How much of the 85% mixture will you need to add to obtain the desired solution?

Answer 1

.05(90)+.85x = .7(90+x)

.15x = .65(90)= 58.5

x =58.5/.15=1170/3= 390 mL

13/3 = x/90

13= x/30

x= 390 mL of 85%