30 men can do a piece of work in 16 days. How many men would be required to do the same work in 20 days ?

Let the required number of men be x
More days, Less men (Indirect proportion)
$$\eqalign{ & \therefore {\text{ }}20:16::30:x \cr & \Leftrightarrow 20x = 16 \times 30 \cr & \Leftrightarrow x = \left( {\frac{{16 \times 30}}{{20}}} \right) \cr & \Leftrightarrow x = 24 \cr} $$