The reasoning in the tutorial is simple, if you have 20 tasks in 3600 seconds = 1 hour, assuming uniform distribution, then the probability of having a task in a particular second will be $20/3600 = 1/180$.
Note that this logic assumes a second can have at most one task. When the number of tasks is small and time resolution is high, this is logical and a simple approximation. Normally, if we are to choose a particular second for each task independent of others, having at least one task at a chosen second is calculated as below, where we also account for collisions: $$1-\left({3599\over3600}\right)^{20}\approx 0.00554\approx\frac{20}{3600}$$