The numbers are not wrong. You are failing to comprehend the geometric nature of the calculation.
The "Cambrian explosion" was millions of years and the Precambrian was longer than the Cambrian. Your students may think they were well instructed, but most of us who also have taught biology are beginning to have our doubts about your claims.
It is you who fail to comprehend my point from two posts back. The thread is about a repetition of an event in different lines of mammals that would appear unlikey to evolve independently. They countered that 4.5 billion years a lot of unlikely things can happen. My reply in part was that the things we are talking about- independent origin of ear bones in mammals and flight in stick insects- are in the higher animals.
Higher animals emerged- with a few possible exceptions, not 4.5 billion years ago but a mere 543 million years ago. All of the independent re-evolution that seems so unlikely happeded since the explosion. That means seven eights of Earth's history is not available to give time for these "unlikely chance events" to occur.