0.9 of 20:
You could also think of 0.9 as 9/10.
9/10, if you multiply by 2/2, is 18/20.
So therefore, you would then know that it would be 18, because 20 was the whole number, so what over 20 is equal to 9/10? 18.
A simpler method is just to multiply. 0.9 * 20 = 18.