Answer: 0.0256 which converts to 2.56%
Explanation:
Assuming each event is independent of one another, the probability of having four people in a row order a salad is 0.40^4 = 0.0256 = 2.56%
If for instance you only wanted to know the probability that the next three people would order salad, then you would compute 0.40^3 = 0.064 = 6.4%