In a race from point X to point Y and back, Jack averages 30 miles/hr to pt Y…

Answer: Sandy

Reasoning:

For a round trip where the outward speed is a and the return speed is b, the average speed over the whole trip is 2ab/(a+b).

• Jack: outward 30 mph, return 10 mph → average = 2×30×10/(30+10) = 600/40 = 15 mph

• Sandy: 20 mph each way → average = 2×20×20/(20+20) = 800/40 = 20 mph

Conclusion: Since Sandy's average speed (20 mph) is greater than Jack's (15 mph), Sandy finishes the round trip first. This conclusion does not depend on the distance of the race.