Correct option is A

Given:

4 boys or 7 girls can complete a piece of work in 18 days. 

Formula Used:

Total work = Number of workers × Efficiency of each worker × Days to complete the work

Solution:

Let the efficiency of each boy and girl be B and G respectively.

According to the concept,

4B = 7G

​$$\frac{B}{G} = \frac{7}{4}$$​​

So, the efficiencies of each boy and girl be 7k and 4k respectively. (k = constant)

Total work = (4 × 7k × 18) units

Now, to complete the job, 12 boys and 21 girls together will take

=> (4 × 7k × 18) ÷ (12 × 7k + 21 × 4k)

=> 3 days