The total ratio representing the slices of pizza that Buzz bought is 5+8=<<5+8=13>>13
If he shared the slices of pizza with the waiter, the waiter received a fraction of 8/13 of the total number of slices, which totals 8/13 * 78 = <<8/13*78=48>>48 slices
Twenty less the number of slices of pizza that the waiter ate is 48-20 = <<48-20=28>>28
#### 28