Starting at home, Ishaan traveled uphill to the gift store for 18 minutes at just 10 mph. He then traveled back home along the same path downhill at a speed of 30 mph. What is his average speed for the entire trip from home to the gift store and back?

The distance to the store is

... (0.3 h) * (10 mi/h) = 3 mi

The time home is

... (3 mi) / (30 mi/h) = 0.1 h

The average speed is the total distance divided by the total time

... speed = (3 mi + 3 mi) / (0.3 h + 0.1 h) = (6 mi) / (0.4 h) = 15 mi/h

Ishaan's average speed was 15 mi/h.