Subject

    Quantitative Aptitude

    Topic

    Time and Work

    Consider three pipes P, Q and R. The pipes P and Q can fill the cistern in 2 and 2.5 hours respectively. The pipe R can empty the cistern in 1 hour. If these pipes were opened at an interval of half an hour at 6:00am, 6:30am and 7:00am respectively. When the cistern will get empty?

    ক)
    7:00 am
    খ)
    10:00 am
    গ)
    12:00 pm
    ঘ)
    2:00 pm
    ঙ)
    1:00 pm

    Explanation

    Amount emptied by pipe R in one hour = 1 As pipe P fills the entire cistern in 2 hours, Part of cistern filled by pipe P in one hour = ½ As pipe Q fills the entire cistern in 2.5 hours, Amount filled by pipe Q in one hour = 1/2.5 For the first half an hour only pipe P is open. ∴ Part of cistern filled by pipe P in starting ½ hour = ½ × ½ = ¼ For the next half an hour, pipe P and pipe Q both are open. ∴ Part of cistern filled by pipe P and pipe Q in next half an hour =12(12+25)=14+15=920= \frac{1}{2}\left( {\frac{1}{2} + \frac{2}{5}} \right) = \frac{1}{4} + \frac{1}{5} = \frac{9}{{20}} ∴ Total pat of cistern filled after one hour =14+920=710= \frac{1}{4} + \frac{9}{{20}} = \frac{7}{{10}} After 1 hour all three pipes are open. Let x be the number of hours for which all 3 pipes are open ∴ At the end of x hours, the cistern will be completely empty 710+x(12+12.5−11)=0\frac{7}{{10}} + x\left( {\frac{1}{2} + \frac{1}{{2.5}} - \frac{1}{1}} \right) = 0 ⇒710−x10=0\Rightarrow \frac{7}{{10}} - \frac{x}{{10}} = 0 ⇒ x = 7 ∴ All three tanks are open for 7 hours. ⇒ Tank will get empty 7 hours after 7:00 am. ⇒ Tank will get empty at 2:00 pm.
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