# seven-gallon jug is full of 30% isopropyl wish to drain some amount and replace with 90% isopropyl to get a solution which 75% isopropyl.?

How many gallons should you drain and

replace?

### 3 Answers

- PhilomelLv 71 month ago
use 1.75G(.3)+5.25G(.9)=7g(.75)

1.75*.3=.525

5.25*.9=4.725

4.725+.525=5.25/7=.75

- az_lenderLv 71 month ago
Let

0.9 x + 0.3 (7 - x) = 0.75 * 7 =>

0.6 x + 2.1 = 5.25 =>

0.6 x = 3.15 =>

x = 3.15/0.6 = 31.5/6 = 5.25.

Drain 5.25 gallons and replace them with the 90% solution.

- StacieLv 41 month ago
Your end result is you want a balance of 90% and 30% to equal 75 percent of the total mixture.

so you need to knowhow much x amount of gallons of 90% and y amount of gallons of 30% are needed first

so (x90+y30)/(x+y)=75, and x+y=7 gallonsso you need to find an average of 75 adding only multiples of 90s and 30sYou can see that (6) parts of 90 + (2) parts of 30 will average 75(6*90=540 and 2*30 = 60, 540+60 = 600, 600/8 = 75)But you need to utilize 7 gallons so there is still more work to be done. You want an end result being 25% of the 7 gallons to be 30% alcoholSo you need to pour out 75% (which is 5.25 gallons) so only 1.75 gallons are left.Then fill up the emptied 75% with 5.25 gallons of 90% alcohol. And this is how you can check:1.75*30=52.55.25*90=472.5475.5+52.5=525525/7=75percent alcohol.