Mario cut a pizza into 9 equal slices he put the slices of pizza on each of 5 plates what is the measure for the angle of the slices that are left

One pizza is 360 degrees.

Cut 9 pieces? divide 9 then. 360/9 = 40 degrees

(I'm a bit confused about your question here but this is how I have interpreted the question)

If we put one slice on each of the plates then: you will have four pieces of pizza left over which is (40 degrees*4pieces) 160 degrees of pizza left.