There is a small difference of numbers used between the graph given in question 2 (3.5$ for personal product) vs the price used in the correction (3$)

On a first look, that seems to be a typo, but @Moritz, who is the author of the case, is better positioned to tell. 

You might also find this useful is you're practicing cases at the moment:

• Cheatsheet: The Must-Know Consulting Terms for Interviews

Best,
Cristian

Yeah, not all cases are 100% correct. When you find these type of inconsistencies when practicing, there isn't an interviewer you can ask which number to use (or why there are two different numbers).

In that case, just assume on of the numbers and move on.