If 72 men can build a wall of 280 m length in 21 days, how many men could take 18 days to build a similar type of wall of length 100 m ?

$$\eqalign{ & {\text{Here work is 280 m length of wall }} \cr & {\text{And 100 m length of wall}} \cr & {\text{Let, M men will finish 100 m wall}}{\text{.}} \cr & \frac{{72 \times 21}}{{280}} = \frac{{{\text{M}} \times {\text{18}}}}{{100}} \cr & \Leftrightarrow {\text{M}} = 30 \cr} $$