Question

In a fort 300 men had provisions for 90 days. after 20 days, 50 men left the fort.how long now would the food last at the same rate?

Answer 1

So men is inversely proportional to days

so with 300 and 90 the constant is 27000

After 20 days, 70 days left and (300*70 = ) 21000 man-days of food left for 250 men.

That will last them (21000/250 =) 84 days.
I think it is clear for you.
If you have any doubts don't forget to ask me.