Answer: The average speed for the entire trip from home to the grocery store and back is 20 mph.
Explanation:
Since we have given that
Ishan traveled uphill to the grocery store for 12 minutes at just 15 mph.
So, Time taken by him = 12 minutes
Speed = 15 mph
So, distance traveled = Speed × time
So, Distance becomes

Again, he traveled back home along the same path downhill = 180 miles
Speed of covering downhill = 30 mph
so, Time taken

Since we know the formula for "Average speed ":

Hence, the average speed for the entire trip from home to the grocery store and back is 20 mph.