In an online media store, downloaded songs cost $3d each and books cost $9b each. Caleb purchased 15 songs and 3 books this month, and also paid $7 in sales tax. Write and simplify an expression to show how much Caleb spent on everything including tax.
Caleb spent (3d + 9b + 7) dollars.
Caleb spent (72d + 7) dollars.
Caleb spent (26b + 45d + 7) dollars.
Caleb spent $79 dollars.