If Dave rents a small boat from a rental service that charges a flat fee of $30.00 plus $10.00 per hour (including taxes). The total charges comes to $90.00. Which equation below can be used to calculate the number of hours x. that Dave rents the boat?

a) 10 x + 30 = 90; 6 hours
b) 10 (x + 30) = 90; 5 hours
c) 30 x + 10 = 90; 6 hours
d) 30 (x + 10) = 90; 6 hours