You know that 9/10 of the class is going to college.
4/5 of those 9/10 will go to a four-year college, while the remaining 1/5 will go to a two-year college.
To determine what part of the class does the 1/5 of 9/10 represent, you have to multiply both fractions:
9/50 of the graduating seniors will be going to a two-year college.