Mixture Problem

Question

You need a 70% alcohol solution. On hand, you have a 260 mL of a 35% alcohol mixture. You also have 90% alcohol mixture. How much of the 90% mixture will you need to add to obtain the desired solution?

You will need how many mL of the 90% solution

Answer 1

90-70 = 20

70 - 35 = 35

35/20 = 7/4

so you need 7/4 times as much 90% as of 35%

x = 7(260)/4 = 7(65) = 455 mL of 90%

260(.35) + x(.90) = (260+x).7

91 +.9x = 182 +.7x

.2x = 91

x = 455 mL of 90%