Question

A man earns rupees 18000 in 3 months. How much time would it take to earn Rs 30000

Answer 1

It would take the man 5 months to earn 30,000 Rupees. If we calculate 18,000 Rupees in 3 months, one month is 6000. Therefore, for 5 months, it is 30,000 as 6000 multiplied by 5 is 30,000. 

Answer 2

We can use the unitary method. 
A man earns Rs. 18000 in 3 months. 
∵Time taken by the man to earn Rs. 18000 = 3 months
∴Time taken by the man to earn Rs. 1 = $\frac{3}{18000}$ months = $\frac{1}{6000}$ months
∴Time taken by the man to earn Rs. 30000 = $( \frac{1}{6000} * 30000)$ months = $( \frac{30000}{6000} )$ months = $\frac{30}{6}$ months = 5 months
∴ It would take 5 months to earn Rs. 30000. 

I am going to show you another method using ratio and proportion. 
The question says that a man earns Rs. 18000 in 3 months. 
Let the time taken by the man to earn Rs. 30000 be $x$ months. 
The units are Rs. and months. There are same units for money and same units for months. 
Now, 
18000 : 30000 : : 3 : $x$
$\frac{18000}{30000} = \frac{3}{x}$
Solve for $x$. 
$\frac{18}{30} = \frac{3}{x}$
Cross multiply. 
$18*x = 3*30$
$18x = 90$
$\frac{18x}{18} = \frac{90}{18}$
$x = 5$
∴ It would take 5 months for the man to earn Rs. 30000. 
If you have done ratio and proportion, you will find this method easier. 

I have shown both the methods. You can try any one method which you will find easier. 

Hope this may help you.