Let N be the total number of alumni
Let A be the number of alumni that attended
The number that expected to attend are made up of those that did attend and those that did not attend. The former is .90*A (90% of the attendees). The latter is .30*(N-A) (30% of the non-attendees)
Therefore, .66*N = .90*A + .30*(N-A)
.66*N = .90*A + .30*N - .30*A
.36*N = .6*A
.6*N = A
So, 60% of the alumni attended.
90% of the attendees said they'd attend, and they did. So, .90*.60=54% of the alumni fall into that category.
70% (100-30) of those who did not attend said they wouldn't. So .70*.40=28% of the alumni fall into that category.
Therefore, 54%+28%=82% did what they said they would do. That means 18% acted differently.