Gunther, the pizza delivery boy, went out from the store carrying pizzas to deliver for the residents in 3 housing societies named Hufflepuff, Ravenclaw, and Slytherin. He started at 6:30 pm from the shop at a speed of 15 mph. After 15 minutes he reached Hufflepuff Society, and it took 15 minutes to deliver all pizzas there. Then at the same speed he moved for another 15 minutes to reach Ravenclaw, and in 15 minutes delivered all pizza. Finally, he drove towards Slytherin at double of his original speed, and after reaching there in 15 minutes time, he realised that the order has been cancelled. What is Gunther's average speed in this whole journey (time includes delivery time)?

A. 8 mph
B. 10 mph
C. 12 mph
D. 15 mph
E. 30 mph

OA C

Source: e-GMAT