In a company 50% of male employees and 40% of female employees are more than 35 years old, 42% of total employees are more than 35 years,ask female employees is what percent of total employees.
The method given by prof is fine and its the one we should stick to in the exam.
Another possible way is notice that taking x*50% and y*40% yields 42%. Well it would always be between 50 and 40 but the distance from 50 tells us what the percentage of females employees is i.e greater the distance from 50, the greater is the %age of female employees.
F% = (50-42)/(50 - 40) = 8/10 = 80%