10 men have ration for 21 days in a camp. If 3 men leave the camp, for how many days will the ration be sufficient for the remaining men? 


Solution: 
Total men = 10 
The men leave the camp = 3 
The remaining men = 7 

We see that as the number of men decreases, the ration will be sufficient for more days (days increase). So it is an inverse proportion. 

Let the number of days be x



As it is a Inverse proportion. So the arrows will be drawn in the opposite direction.

Now the the proportion will be 

  7 : 10  :: 21 :  X 

As we know that the product of mean is equal to product of extreme of a proportion we have

 10 ï½˜ 21  = 7 ï½˜  X 

or

 X   =  `\frac{10 ｘ 21}{7}`

by simplifying 

 X   =  30 days

Thus, the Food (Ration) will be sufficient for 30 days