A mid-size city must decide whether or not to build a new combined bus and train station. To build the new station will require an increase in city taxes. According to a city politician, 70% of all city residents support the tax increase to build a combined bus and train station. An opinion poll of 400 city residents will ask whether they favor a rise in taxes to pay for a combined bus and train station. What is the standard deviation of the distribution of sample proportions?