Correct Answer - Option 2 : 12

**Given-**

Time taken by Arun to complete the work = 36 hours

Ballu is twice as efficient as Arun

**Formula Used-**

Total Work = Efficiency × Total Time

**Calculation-**

Part of work done by Arun in 1 hour = 1/36 [If total work is assumed to be 1]

Part of work done by Ballu in 1 hour = 2 × 1/36 ∵ Ballu is twice as efficient as Arun

⇒ 1/18

Part of work done by Arun and Ballu in 1 hour = 1/36 + 1/18

⇒ 3/36

⇒ 1/12

∴ Time taken by both of them to complete the whole work = 1/(1/12)

⇒ 12 hours