A garrison of 900 men had provisions for 42 days. However, a reinforcement of 500 men arrived. For how many days will the food last now?
More the number of men, lesser the days to feed them by given food. So, it is an inverse proportion.
Let the required days be x.
⇒ 900 × 42 = 1400 × x
⇒ x = 
= 27 days
12