36 men can complete a piece of work in 18 days. In how many days will 27 men complete the same work ?

Less Men, means more Days {Indirect Proportion}
Let the number of days be x then,
27 : 36 :: 18 : x

[Please pay attention, we have written 27 : 36 rather than 36 : 27, in indirect proportion, if you get it then chain rule is clear to you :)]                                

=>27x = 36 * 18

=> x = 24

 So 24 days will be required to get work done by 27 men.