It is a binomial distribution probability problem with the number of trials n=20 and individual success probability p=0.75. Without writing long formulas, I refer to this source https://www.sjsu.edu/people/saul.cohn/courses/stats/s0/BinomialProbabTable.pdf which gives the tables of calculated probabilities for different values of n and p. Find there the table for n=20 and p=0.75. It is what you ask for, in the form ready for consumption.