Assignment:
A door-to-door sales person for a household appliance has learned from her past experience that out of ten demonstrations of her appliance only four result in actual sales (long run average). This week she needs to make at least five sales. At least how many demonstrations does she need to perform to ensure that the probability of meeting her target is at least 90 percent? Try to answer this question playing with cumulative probabilities.
Provide complete and step by step solution for the question and show calculations and use formulas.