Problem: on a certain college faculty, (4/7) of the professors are male, and the ratio of the professors older than 50 years to the professors less than or equal to 50 years is 2:5. If (1/5) of the male professors are older than 50 years, what fraction of the female professors are less than or equal to 50 years?

I was able to find (female and less than or equal to 50 yrs) which is (9/35). The official solution involved dividing this fraction by (3/7). I don't quite understand the extra step of dividing 9/35 by 3/7.