What was the number of employees at the end of the year 2017?
Answer: C
This is a cumulative calculation.
End of 2015: 500 employees.
Start of 2016: 500 employees. End of 2016 = 500 + 100 - 50 = 550 employees.
Start of 2017: 550 employees. End of 2017 = 550 + 150 - 80 = 700 - 80 = 620 employees.
The number of employees at the end of 2017 was 620.