Answer: If the army regiment had 400 soldiers with provisions for 80 days, the food would last each soldier for 80 days / 400 soldiers = 0.2 days per soldier.
If 80 soldiers moved out, the number of soldiers remaining in the regiment would be 400 soldiers - 80 soldiers = 320 soldiers.
Since the amount of provisions has not changed, the food would last for 80 days / 320 soldiers = 0.25 days per soldier.
Therefore, the food would last the 320 soldiers for 0.25 days * 320 soldiers = 80 days.
Explanation: