<PRE><B>Question 628315</B>
A pipe that takes 30 minutes to fill a tank has been running to 
fill an empty tank.
 After 10 minutes, it is shut off and then a second pipe was opened and was
 able to fill the tank in 15 minutes.
 How long will it take the second pipe alone to fill the tank?
:
Let x = time for the 2nd pipe to fill the tank alone
Let the completed job = 1, (a full tank)
:
Each pipe will fill a fraction of the tank, the two fractions add up to 1.
:
{{{10/30}}} + {{{15/x}}} = 1
multiply by 30x, results:
10x + 30(15) = 30x
450 = 30x - 10x
450 = 20x
x = {{{450/20}}}
x = 22.5 minutes the 2nd pipe alone
:
:
Check this
10/20 + 15/22.5 =
1/3 + 2/3 = 1
</PRE>