According tot he question,

A takes x and B takes 3x days to finish the job

 ⇒ 3x - x=30

∴ x=$\frac{30}{2}$=15 days

Time taken by B to complete the work = 3x = 3×15= 45 days

In 1 day (A+B) can complete$=\frac{1}{15}+\frac{1}{45}=\frac{3+1}{45}=\frac{4}{45}$

∴ (A+B) will finish the work in $\frac{45}{4}=11\frac{1}{4}$days

Hence option C is the correct.