Gina Fox has started her own company, Foxy Shirts, which manufactures imprinted shirts for special occasions. Since she has just begun this operation, she rents the equipment from a local printing shop when necessary. The cost of using the equipment is $250. The materials used in one shirt cost $10, and Gina can sell these for $15 each. How many shirts must Gina sell to break even?
a) 70 shirts
b) 40 shirts
c) 50 shirts
d) 80 shirts