Explanation

10 men have ration for 21 days, which means the total ration is sufficient for 10 x 21 = 210 man-days.

If 3 men leave the camp, the remaining number of men is 10 - 3 = 7 men.

The total ration remains the same, but now it needs to be distributed among 7 men instead of 10.

To find the number of days the ration will last for the remaining men, divide the total ration (210 man-days) by the number of remaining men (7): 210 ÷ 7 = 30 days.

Therefore, the correct answer is: 30 days