Anna wants to rent movies from either service a or service b. service a charges $37.92 as a subscription fee with a charge of $2.18 per movie. service b charges $33.25 as a subscription fee plus $3.55 per movie. service b is running a special offer where the first movie is free. given that anna always rents at least 1 movie in a given subscription period, how many movies would she have to rent for the average cost per movie to be equal at service a or service b?