During 2013, Joule Company, a sole proprietorship, earned general business tax credits of $30,000 for energy conservation and rehabilitation expenditures. The owner, Mark Joule, knows that the overall credit will be limited. He provides you with the following information for 2013: regular tax (before credits) = $37,500; tentative minimum tax = $20,200; alternative minimum tax = $0; foreign tax credit = $4,500; other nonrefundable credits  $6,100. Mark also has a $3,000 carryforward of general business credit from 2012.
a. What is the 2013 limitation on Mark's general business credit?
b. What is Mark's allowable general business credit in 2013 and where does it come from?
c. What is Mark's general business credit carryover?