Question

The sum of 16 and a number x is equal to 20

Answer 1

" the sum " means add

16 + x = 20 <= ur equation
x = 20 - 16
x = 4 <== the number

Answer 2

The sum of 16 and 4 is equal to 20.
This is how I know:
20-16=4
How do I know this? 
Sum means to add. To get the sum of something that equals another number, you must see what number adds up with 16 to get 20. So, this means that 16+4=20.

Hope this helps. <3 :)