You can put this solution on YOUR website! As I understand this
240 men had a provisions for 21 days,
for how many men will provisions be sufficient for 18 days?
:
240 * 21 = 5040 man-days of provisions available
let m = the no. of men provided for in 18 days
18m = 5050
m = 5040/18
m = 280 men