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A fish tank in a pet store was less than half full of water
when a worker turned on a hose to fill the tank at a constant rate.
After the hose was on for 5 minutes, the tank was exactly half full.
After the hose was on for 30 minutes, the tank was three-quarters full.
How many minutes did it take after the hose was turned on for the tank
to be completely filled with water?
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It is a joke Math problem, and it can be solved in couple of lines
using simple and elementary reasoning INSTEAD of using complicated formulas.
In 30-5 = 25 minutes, the hose fills the tank from 1/2 to 3/4.
In other words, it takes 25 minutes to fill 1/4 of the tank.
It means that in 5 + 25 + 25 = 55 minutes from the start, the tank will be filled in full.
Solved, completed and explained.
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The meaning of this problem is to give to a person an opportunity to find simple/simplest solution.
In such form, it is accessible to any 3th grade young student,
and it REALLY teaches him (or her) to think in a right way and do not make unnecessary job.
It is also good for those older than 70 as an entertainment problem.
I would say, it also good as a blitz-test problem at the job-hiring interview
to determine if a person is able to think creatively.
Let god save you from solving it in this way as @josgarithmetic proposes to do it in his post.
Doing that way in the interview, you will 100% fail such blitz-test.