Final answer:
Approximately 30 trucks, each with a 3.5-ton payload, would be needed to unload the same freight train.
Step-by-step explanation:
To unload a freight train with 2.5-ton payloads, 21 trucks would be needed. To find out how many trucks with 3.5-ton payloads would be needed to do the same job, we can set up a proportion.
Since the number of trucks is inversely proportional to the weight of the payloads, we can use the following proportion:
2.5 tons : 3.5 tons = 21 trucks : x trucks
Cross-multiplying, we get 2.5x = 3.5 * 21. Solving for x, we find that x is approximately 29.4.
Therefore, approximately 29 or 30 trucks, each with a 3.5-ton payload, would be needed to unload the same freight train.