Suppose you defined ""positive"" motion as motion westward. What would be the persons total velocity?

I think that you are asking about an interactive reader which talks about an exercise which asked for the person's velocity who were on the bus.

The answer is 14 m/s

Explanation: When you have to do an exercise about velocity, you can do this exercise knowing the velocity from the bus in relative the street and you ALWAYS have to define your positive or negative motion about the direction in which the person were walking to know if you have to add or subtract

In this case, the bus were travelling east at 15 m/s and the person walked toward the back of the bus at 1 m/s.

It doesn't matter if the positive is the west or the east. While you only know the positive and knowing that the person was walking toward back the bus you can deduce this:

The person's total velocity is (+15 m/s) - (+1 m/s) = +14 m/s