How large a sample do you need to take to ensure that you can estimate, with 90% confidence, the proportion of people preferring Coke to within 5% of the actual value?

Problem – You have been assigned to determine whether more people prefer Coke or Pepsi. Assume that roughly half the population prefers Coke and half prefers Pepsi. How large a sample do you need to take to ensure that you can estimate, with 90% confidence, the proportion of people preferring Coke to within 5% of the actual value?