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    Question

    A, B, and C can fill a tank in 12, 15, and 20 minutes

    respectively. A starts alone; after 3 minutes B joins; after another 2 minutes C joins. How long will they take to fill the tank?
    A 7.25 mins Correct Answer Incorrect Answer
    B 12.55 mins Correct Answer Incorrect Answer
    C 19.2 mins Correct Answer Incorrect Answer
    D 29.5 mins Correct Answer Incorrect Answer

    Solution

    ATQ,

    Work per min: A=1/12, B=1/15, C=1/20 3 min тЖТ A: 3/12 = 1/4 Next 2 min тЖТ A+B: 2├Ч(1/12+1/15)=2├Ч(9/60)=3/10 Total = 1/4 + 3/10 = 5/20 + 6/20 = 11/20 Remaining = 9/20 All three тЖТ per min = (1/12+1/15+1/20)= (5+4+3)/60 = 12/60=1/5 Time = (9/20)/(1/5)= (9/20)*5=9/4=2.25 min Total = 3+2+2.25 = 7.25 minutes.

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