In this video the following example is about finding the critical value in the upper tail.

A particular drug has a 1 in 4 chance of curing a certain disease. A new drug is developed to cure the disease. How many people would need to be cured in a sample of 20 if the new drug was to be deemed more successful at curing the disease than the old drug to obtain a significant result at the 5% level?

Hypothesis testing - Finding an Upper Critical Value : Binomial Distribution : ExamSolutions - youtube Video