The U.S. Federal Reserve System publishes data on family income based on its Survey of Consumer Finances. When the head of the household has a university degree, the mean pre-tax family income is US $85,200. Suppose that 60% of the pre-tax family incomes when the head of the household has a university degree are between US $75,600 and US $94,800 and that these incomes are normally distributed. What is the standard deviation of pre-tax family incomes when the head of the household has a university degree?