Let A take a and B take 3a days to finish the work.

As per the question,

3a-a=30

 $\therefore \mathrm{x}=\frac{30}{2}=15 \mathrm{days}$

Time taken by B to complete the work is 3a

So 3×15=45 days

Therefore 1 day work of (A+B)= $\frac{1}{15}+\frac{1}{45}=\frac{3+1}{45}=\frac{4}{45}$

Hence, (A+B) will finish the work in $\frac{45}{4} \mathrm{or} 11\frac{1}{4}\mathrm{days}$

Hence option C is correct