Wackerly is not the ideal book for a first introduction to mathematical statistics for non-mathematics majors, but for the motivated upper-level undergraduate or graduate mathematics student, it is an acceptable choice.  Like most mathematical textbooks intended for that audience, the authors assume that the reader will get out a pencil and paper and work through whatever "missing steps" or gaps in explanation the reader needs to fill in.  There is enough in this textbook so that, if the reader reads carefully, the reader should be able to fill in any missing steps.