M can do a piece of work in 12 days and N in 24 days. They begin together, but M leaves the work 6 days before the completion of the work. Find the total number of days to complete the entire work.

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  1. 10 days
  2. 12 days
  3. 7 days
  4. 6 days

Answer (Detailed Solution Below)

Option 2 : 12 days
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Detailed Solution

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Given:

M can do the work in 12 days.

N can do the work in 24 days.

M leaves 6 days before completion.

Concept/Formula used:

Work done = Efficiency × Time.

Efficiency here is the amount of work done by M and N in one day.

Solution:

Calculate LCM of 12, 24

⇒ LCM = 24 (total work)

Efficiency of M:

⇒ 24/12 = 2 units/day

Efficiency of N:

⇒ 24/24 = 1 unit/day

Combined efficiency of M and N:

⇒ 2 + 1 = 3 units/day

Work done by N alone in the last 6 days:

⇒ 1 unit/day × 6 days = 6 units

Remaining work (done by M and N):

⇒ 24 units - 6 units = 18 units

Time for M and N to do remaining work:

⇒ 18 units ÷ 3 units/day = 6 days

Total time =  6 days (M and N together) + 6 days (N alone) = 12 days

Hence, the total time to complete the work is 12 days.

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