Math homework questions.?

Question

I need help solving these two problems.
1)How many liters of water should be evaporated from 160liters of a 12% saline solution so that the solution that remains is a 20% saline solution?

2)Printer A can print a report in 3 hours, printer B can print the same report in 4 hours. How long will it take both printers working together, to print the report?

Answer 1

Stef, got to look for the principle behind the problems.
In #1, evaporation does not remove any salt. We use this in setting up the problem. At any time,
weight of salt/liters of solution= solution strength.
Let weight = x. Then at the start, (x/160)=0.12 and x=19.2
Let final volume=y, then 19.2/y=0.20 y=98 liters (appx)
Amount evapoated = 160-98=62 liters.
There are other ways to set this up, but this is simple to follow.

2. Suppose the report is P pages long.
Then for printer A: speed is P/3 pages/hour
for printer B: speed is P/4 pages/hour
If both printers work together,
P = (speed of each printer) x time. Let time=t
P= Pt/3+Pt/4 or 1= t/3 + t/4. Then 12=7t and t=12/7 hours. Note that the P drops out. Moral, don't be afraid to use more than one unknown in a problem. The principal is the thing.

Answer 2

report/3 hours + report/4 hours=
4 reports/12 hour + 3 reports/12 hours=
7 reports /12 hours=
1 report / 12/7 hours ~ 1 report in about 1.7142857142857142857142857142857 hours
In 12 hours the first printer would print 4 reports.
In the same 12 hours the second printer could print 3 reports.
That would be a total or 7 reports in 12 hours or 1 report in 12/7 hours.

Answer 3

2)

1report/3hrs(printer A)+1report/4hrs(printer B)=report/hrs can print together


4r/12+3/12=7report/12hrs

1:x(hrs to finish the report together)=7:12

12=7x
x=12/7hrs
or

1.714 hrs

Answer 4

DEPEND ON WHICH PRINTER YOU SEND THE ITEM TO.
IF you slit the report it will take 31/2 hours

Answer 5

small error in the answer above.

19.2/.2 = 96

160-96 = 64