A, B and C can do a piece of work in 15, 10 and 30 days, respectively. In how many days can A do the work if he is assisted by B and C on every 3rd day? 

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  1. 9 days 
  2. 12 days 
  3. 6 days 
  4. 15 days 

Answer (Detailed Solution Below)

Option 1 : 9 days 
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Detailed Solution

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Given:

Time taken by A = 15 days

Time taken by B = 10 days

Time taken by C = 30 days

B and C both assist A on every third day

Formula used:

Work = Efficiency × Time

Calculation:

LCM of 15, 10, and 30 is 30

Efficiency of A = 30/15 = 2 units/day

Efficiency of B = 30/10 = 3 units/day

Efficiency of C = 30/30 = 1 unit/day

Work done by A on the first two days = (2 × 2) = 4 units

Work done by A, B, and C together on the third day = (2 + 3 + 1) = 6 units

Total work done by A, B, and C in 3 days = (4 + 6) = 10 units

Similarly, work done by A, B, and C in the next three days = 10 units

Total work done by A, B, and C in 6 days = (10 + 10) = 20 units

Remaining work = 30 - 20 = 10 units

Total work done by A, B, and C in naxt days = 10 units

Number of days taken by A to complete the remaining work = 3 + 3 + 3 = 9 days

∴ The required time is 9 days.

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