6.69 Global warming Suppose that weekly use of gasoline for motor vehicle travel by adults in North America has approximately a normal distribution with a mean of 20 gallons and a standard deviation of 6 gallons. Many people who worry about global warming believe that Americans should pay more attention to energy conservation.

Assuming that the standard deviation and the normal shape are unchanged, to what level must the mean reduce so that 20 gallons per week is the third quartile rather than the mean?