Two cars leave a gas station at the same time, one traveling north and the other south. The northbound car travels at 50 mph. After 3 hours the cars are 345 miles apart. How fast is the southbound car traveling?

Traveling in opposite directions, the cars’ speeds add to give their rate of separation. They are 345 miles apart after 3 hours, so their combined speed is 345 ÷ 3 = 115 mph. Subtract the northbound car’s 50 mph: the southbound car travels 115 − 50 = 65 mph, choice b, 65 mph.

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