Question

Food express is running a special promotion in which customers can win a free gallon of milk with their food purchase if there is a star on the receipt so far 147 of the first 156 customers have not received a star in the receipts what is experimental probability of winning a free gallon of milk

Answer 1

Formula
P(nw) = no wins / total
no wins = 147
Total = 156

Calculation for no wins
P(nw) = 147 / 156 = 0.9423

Calculation for winners.
P(w) = 1 - 0.9423
P(w) = 0.05769

Another way.
You could calculate the winners from the total
156 - 147 = 9

P(w) = 9/156 = 0.05769 Same answer as before. Pick the method you like best.