Java if statement to change a varible value : I've just stated Java, roughly done 5-6 hours but I have no idea how to approuch this task.

I'm doing a tax system for a hotel. 
A standard rate of tax is used (20%). 
However, the program askes the user if they want to change this rate by asking the user to input "Yes" or "No", if yes, proceed to enter the new value of the Tax Varible, if no, it keeps the standard rate of (20%).

There's a lot more to the task. However, this is the only issue I'm struggling with. 

Can anyone tell me how to write such an if statement? I have absolutely no idea how to do it with such specific user input.

Any sort of help or suggestions would be appreciate so I can play around with it.
