Explain how to use mental math to find 620-278?

What I did was add 22 in my head to 278 to equal 300.

I then just took 620-300 = 320.

Then don't forget about the 22 you added earlier because now you need to add it to the total. 

320+22= 342

OR

You can add 42 to the 278 to give you 320.

Then you can easily see that its 620-320 = 300.

Then add the 42 to the 300 to give 342

Idon’t understand what you’re asking..