My instructor made this question up But we use the Second Ed

My instructor made this question up. But we use the Second Edition, Intermediate Algebra testbook, by Bittinger, ellenbogen, Johnson and chapter 6 - sorry if you have answered a question like this already? I saw some similar but not with the \"skill water\"...
question is.
A boat travels 10 miles per hour in still water. It travels 20 miles down a river in the same time that it travels 5 miles up the same river. What is the speed of the River?
thank you.

Solution

20/(10+X)=5/(10-X) CROSS MULTIPLY
5(10+X)=20(10-X)
50+5X=200-20X
5X+20X=200-50
25X=150
X=150/25
X=6 MPH FOR THE CURRENT
PROOF
20/16=5/4
1.25=1.25
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NEW PROBLEM: THESE ARE THE VERY EASY ONES
X*Y/(X+Y)=TOTAL TIME WHERE X & Y ARE THE TIMES IT TAKES EACH ONE INDICVIDUALLY TO DO SOMETHING OR WVWN PIPES TO FILL/EMPTY A POOL.
IN YOU PROBLEM WE HAVE A 40 MINTE JOB & AN UNKNOWN JOB TIME THAT ENDS UP BEING A 15 MINUTE JOB IF COMBINED. THUS LET X BE THE UNKNOW TIME.
40X/(40+X)=15 NOW CROSS MULTIPLY
40X=15(40+X)
40X=600+15X
40X-15X=600
25X=600 X=600/25
X=24 MINUTES FOR THE OTHER PERSON TO DO THE JOB ALONE.
PROOF
40*24/(40+24)=15
960/64=15
15=15