Hi there! So, we will find the LCD by finding the LCM of both mulitples, which is the lowests number that both of them have in common when being mulitplied by a number. Let's list them out.
16: 16, 32, 48, 64, 18
20: 20, 40, 60, 80, 100
As we can see, they both have the number 80 in common. That number will be our LCD. Let's convert. 16 goes into 80 five times and 5 * 5 is 80. 5/16 is 25/80. 20 goes into 80 four times and 6 * 4 is 24. That makes 24/80. The expression is 25/80 + 24/80. Add both of those numbers and the sum is 49/80.