Question 1099899
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{{{1/20}}} of the job per minute is the rate of the 1-st tube.

{{{1/30}}} of the job per minute is the rate of the 2-nd tube.

{{{1/60}}} of the job per minute is the rate of the 3-rd tube.

The combined rate of work of all three tubes working together is  

    {{{1/20 + 1/30 + 1/60}}} = {{{3/60 + 2/60 + 1/60}}} = {{{6/60}}} = {{{1/10}}} of the job per minute.


Hence, it will take 10 minutes for the three tubes to fill the tank working together.
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