SOLUTION: If pump A can empty a tank in 2 hours and pump B can empty the tank in 4 hours, how long will they take to empty the tank if they run together.

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Question 941893: If pump A can empty a tank in 2 hours and pump B can empty the tank in 4 hours, how long will they take to empty the tank if they run together.
Found 2 solutions by josmiceli, macston:
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Add their rates of emptying
pump A:
( 1 tank emptied ) / ( 2 hrs )
pump B:
( 1 tank emptied ) / ( 4 hrs )
A and B together:
( 1 tank emptied / ( t hrs )
--------------------------
+1%2F2+%2B+1%2F4+=+1%2Ft+
Multiply both sides by +4t+
+2t+%2B+t+=+4+
+3t+=+4+
+t+=+1+%2B+1%2F3+ hrs
and
+1%2F3+ hr = +20+ min
-------------------------
Running together, A and B empty
the tank in 1 hr 20 min
----------------------
check:
+1%2F2+%2B+1%2F4+=+1%2F%28%284%2F3%29%29+
+1%2F2+%2B+1%2F4+=+3%2F4+
+3%2F4+=+3%2F4+
OK

Answer by macston(5194) About Me  (Show Source):
You can put this solution on YOUR website!
Pump A empties 1 tank/2hrs=1/2 tank per hr
Pump B empties 1 tank/4hrs=1/4 tank per hr
Pump A+Pump B=(1/2+1/4)tank per hour =3/4 tank per hr
1 tank/3/4 tank per hour=4/3 hour or 80 minutes