Analysis of bottled water. Refer to the report on whether bottled water is really purified water, presented in Exercise 4.72 (p. 240). Recall that the Natural Resources Defense Council found that 25% of bottled-water brands fill their bottles with just tap water. In a random sample of 70 bottled-water brands, let x be the number that contain tap water.

a. Find the mean of x.

b. Find the standard deviation of x.

c. Find the z-score for the value x = 25.

d. Find the approximate probability that 25 or more of the 70 sampled bottled-water brands will contain tap water.