Correct Answer - Option 3 : 18 days

**Given:**

P can complete the work in 36 days

Q can complete the same work in 12 days

**Concept used:**

Total work = Efficiency × Time period

**Calculation:**

Taking the L.C.M of 36 and 12

L.C.M of 36 & 12 = 36

⇒ One day work of P = 36/36 = 1

⇒ One day work of Q = 36/12 = 3

⇒ Work completed in two days = 1 + 3 = 4

⇒ Work completed in one day (Alternatively) = 4/2 = 2

⇒ Number of days to complete the whole work = 36/2 = 18

**∴ The number of days to complete the whole work is 18 days.**