"The ratio of the number of male staff to the number of female staff is 2:3."
This means if
is the number of male staff, and
is the number of female staff, then these numbers occur in a fixed ratio such that
The sentence does not say anything about the actual number of staff members. There's no reason to believe that there are 50 people working.
What it does say is that 2/5 = 40% of the team is made of males and 3/5 = 60% is made of females.
(a) Of the males, 20% = 1/5 of them are under the age of 20. This means that 20% of 40% of the entire staff is made up of males under 20, or
2/5 × 1/5 = 2/25 = 8%
Of the females, 40% = 2/5 are under the age of 20. So 40% of 60% are females under 20, which comes out to
3/5 × 2/5 = 6/25 = 24%
Adding these up, we get a total of 8% + 24% = 32%.
(b) Yes, any change to the team would adjust this percentage.
Consider a smaller example of a team of 4, 1 each of male/female and under/over 20. 1/4 = 25% of this team is female and under 20. If we add one more under-20 female, the new ratio is 2/5 = 40%. If we add one more member to any other group, the new ratio is 1/5 = 20%.