SOLUTION: Add and write in simplest form: 5/4cd+11/4cd= my answer is 16/8cd am I right?

No.  You are confusing what you do when you MULTIPLY fractions
with what you do when you ADD factions.

To MULTIPLY fractions you MULTIPLY tops and bottoms.

To ADD fractions which have a common denominator, you ADD the tops ONLY.
You DO NOT add the bottoms.  You merely KEEP the common denominator.



Now add the numerators, and put it over the common denominator



or



Now the 4 in the bottom will cancel into the 16 on top
giving 4 on top and the final answer is



Edwin