Susan and Stan Britton are a married couple who file a joint income tax return, where the tax rates are based on the tax table 3.5. Assume that their taxable income this year was $390,000. Do not round intermediate calculation. a. What is their federal tax liability? Round your answer to the nearest dollar. 5 b. What is their marginal tax rate? Round your answer to the nearest whole number. 06 c. What is their average tax rate? Round your answer to two decimal places. %