7.70 For itemized tax returns in the $60,000–$75,000 income group for the most recent year reported, the average charitable contribution was $1935. Assume a normal distribution and a standard deviation of $400.

Source: Statistical Abstract of the United States 2002, p. 360.

a. For a randomly selected household from this income group, what is the probability that the household made charitable contributions of at least $1600?

b. For a randomly selected household from this income group, what is the probability that the household made charitable contributions of between $2200 and $2400?

c. What level of contributions was exceeded by only 20% of the itemized returns in this income group?

d. What level of contributions was exceeded by 40% of the itemized returns in this income group?