Question 100986
First add all the pieces together to find the total length Darren 
used when framing the doorway.
7 1/2 + 7 1/2 + 2 3/4 
15 + 2 3/4
17 3/4 feet
Now if Darren started with 20 feet of moulding. We can find out how 
much is left over by subtraction.
20 - 17 3/4
2 1/4 feet 
<b>Answer: So there is 2 1/4 feet of moulding left over.</b>


Side note: It may be interesting to note that in real world application the left over amount would be slightly shorter than 2 1/4 feet. This is because if you were to actually take a saw and cut a piece of moulding a small part of the moulding equal to the width of the blade will be lost. Essientially turned into saw dust.