If 30 oxen can plough 1/7 th of a field in 2 days, how many days 18 oxen will take to do the remaining work?

Answer: Option B

We will use work equivalence method,
$$\begin{array}{rl}& \frac{30}{18}=\frac{\frac{1}{7}}{\frac{6}{7}}\times \frac{x}{2}\\ & \frac{5}{3}=\frac{1}{6}\times \frac{x}{2}\\ & Or,x=\frac{60}{3}=20\text{days}\end{array}$$