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Anu, Vinu and Manu can complete a work alone in 15 days, 12 days and 20 days, respectively. Vinu works everyday. Anu works only on alternate days starting from the first day while Manu works only on alternate days starting from the second day. Then, the number of days needed to complete the work is

Solution

✅ Correct Option: 2

When someone can complete work in nn days, their daily work rate =1n= \frac{1}{n} of total work.

Anu's rate =115= \frac{1}{15} per day

Vinu's rate =112= \frac{1}{12} per day

Manu's rate =120= \frac{1}{20} per day


Vinu works every day (Days 1, 2, 3, 4, 5, 6...)

Anu works on alternate days starting Day 1 (Days 1, 3, 5, 7, 9...)

Manu works on alternate days starting Day 2 (Days 2, 4, 6, 8, 10...)


Day 1: Anu + Vinu work together

Work done =115+112=460+560=960= \frac{1}{15} + \frac{1}{12} = \frac{4}{60} + \frac{5}{60} = \frac{9}{60}


Day 2: Vinu + Manu work together

Work done =112+120=560+360=860= \frac{1}{12} + \frac{1}{20} = \frac{5}{60} + \frac{3}{60} = \frac{8}{60}


Every 2 days, total work completed:

=960+860=1760= \frac{9}{60} + \frac{8}{60} = \frac{17}{60}


After 6 days (3 complete cycles):

Work completed =3×1760=5160= 3 \times \frac{17}{60} = \frac{51}{60}

Remaining work =1−5160=960= 1 - \frac{51}{60} = \frac{9}{60}


Day 7: Anu + Vinu work together

Work done =960= \frac{9}{60}

Total work after Day 7:

=5160+960=6060=1= \frac{51}{60} + \frac{9}{60} = \frac{60}{60} = 1


The work will be completed in 7 days.

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