Final answer:
Gary's average speed on level ground is 19.5 miles per hour.
Step-by-step explanation:
Step 1: Calculate the total time spent on each type of terrain. Gary spent 2 hours on level ground and 5 hours on uphill terrain, totaling 7 hours.
Step 2: Calculate the total distance covered. Gary hiked a total of 39 miles.
Step 3: Set up the equation. Let x be Gary's speed on level ground and x - 3 be his speed on uphill terrain. The equation can be written as 2x + 5(x - 3) = 39.
Step 4: Solve the equation. Simplify and solve to find x = 8.
Step 5: Find the average speed on level ground. The average speed is the total distance divided by the total time, so the average speed on level ground is 39 miles divided by 2 hours, which is 19.5 miles per hour.