Find water left in a tank using arithmetic sequences

by Problem posted Whitney Hamilton
(Edmond, OK, United States)

A water tank is emptied at a constant rate. Initially, 36,000 gallons of water were in the tank. A the end of five hours, 16,000 gallons remained. How many gallons of water were in the tank at the end of the third hour?


In order to get to 16000 gallons, we have to subtract a number. Since this number is constant, it is the same number we subtract each time.

Let us call this number we subtract n.

First hour: 36000

Second hour: 36000 - n

Third hour: 36000 - n - n

Fourth hour: 36000 - n - n - n

Fifth hour: 36000 - n - n - n - n

At this point, what is left in the tank is
36000 - n - n - n - n

It is also equal to 16000 as stated in the problem.


36000 - n - n - n - n = 16000

36000 - 4n = 16000

36000 - 16000 = 4n

20000 = 4n

20000/4 = 4n/4

5000 = n

Therefore, the number to subtract each time is 5000

Third hour: 36000 - 5000 - 5000 = 26000

There are 26000 gallons of water in the tank at the end of the third hour.

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Jul 26, 2017
Really Helpful Article NEW
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Jul 24, 2017
Let n= numbskull doh ! NEW
by: Anonymous

Your kidding , right?

Jul 23, 2017
My opinion NEW
by: Aada

It should be 24000 gallons left after 3hours

Jul 21, 2017
Old School Figuring NEW
by: Anonymous

This starts off by saying that after 5 hours, there is 20,000 gallons less in the tank.
Clearly the draw is at 4,000 gallons per hour.

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