Correct Answer - Option 1 : 21 days

**Given:**

Time taken by 12 machines working 7 hours a day = 18 days

**Formula used:**

(M1 × D1 × H1)/W1 = (M2 × D2 × H2)/W2

**Calculation:**

By using the above formula for calculating the no of days:

(12 × 18 × 7)/1 = (16 × D2 × 9)/2

⇒ 12 × 18 × 7 × 2/(16 × 9) = D2

⇒ D2 = 21 days

**∴ 16 machines working 9 hours a day finish twice the work in 21 days. **