If you think of directions, East and West are 180 degrees apart from each other. In other words, going West after traveling East is like turning around and walking back.

So Sheldon actually is 20 miles towards West of the original location from where he started. He droves 15 miles to the East and then drove 35 miles West from that location. This means out of his 35 miles drive to the west, 15 miles drive brings him back to his original location and then 20 miles (35-15 miles) is towards the west of his original location.