a) 51 out a 100 students will graduate in six years or less

b) 49 out of a 100 students who will graduate later or never.

Since 25 out of 100 are drop-outs that means than from the set of 49, 25 will never graduate and the others 24 will finish after six year. In summary you have out of population of 100 the following sets:

Set A 51 students graduating before six years

Set B 24 students graduating after six years

Set C 25 students never graduating .

So you are right to conclude that if a student is retained has a chance 51/(51+24) to graduate within the six years of enrollment.