Draining The Swimming Pool
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Byrne, William Martin Luther King H.S.
To do some experiments that suggest a way to solve the general work problem:
If A can do a job in X hours and B can do the same job in Y hours, how long will it
take A and B working together to complete the job?
More specifically, if one pipe can drain a swimming pool in 4 hours and another pipe
can drain the pool in 7 hours, how long will it take to drain the pool using the two
Plastic container holding 2 or 3 liters, several plastic tubes of different inside
diameter ( eg. 1/2", 1/4", 3/16" ) each about 4 feet long, stopwatch or clock with
second hand, several pails for holding water.
Use overhead to show fractional parts of container drained in a given number of
The equation given in textbooks to solve the swimming pool problem is usually
stated as X/4 + X/7 = 1. It is not immediately obvious why this formula works or how
it was derived. The following is a strategy for leading up to the formula.
Determine the number of seconds or minutes it takes to drain the same amount of
water from the plastic container using each of the plastic tubes separately. You can
ask for student volunteers, one to hold a tube and another to siphon the water.
Students at their desks can use the clock to determine the times. Have the students
write down the times next to the i.d. of the tubes. Many questions can be raised
along the way. For example what is the relationship between the i.d. of the tube and
the draining time for that tube? If the radius of the tube is doubled what happens
to the inside area of the tube, the amount of water it will carry and thus the
draining time? Make estimates and then test them using the appropriate tube. An
important question is what fraction of the container is drained by one or more tubes
in 1 sec. Suppose the draining times for two tubes are 25 and 40 sec. Then in 1
sec., (1/25 + 1/40 ) of the container will be drained. Ask the students to find a
proportion using this idea. Hopefully after a few minutes working in groups they
will come up with this proportion: (1/25 + 1/40) / 1 sec = 1 / X sec , where X is the
time to drain using two tubes at the same time. Now choose two tubes, do the
experiment and see if the time you get agrees with the solution you get using the
equation. The equation given at the beginning follows from the proportion.