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Alice leaves her house, driving east at 45 miles per hour (mph). Thirty minutes later, her husband Dave notices she forgot her cell phone and sets off after her. How fast must Dave travel in order to catch up with Alice 3 hours after he leaves?

A 52.5 mph

By the time Dave leaves, Alice has already been traveling for half an hour. Three hours later, she would’ve been traveling for 3.5 hours at 45 mph, or 157.5 miles. In order to travel 157.5 miles in 3 hours, Dave would have to travel at 52.5 mph (divide 157.5 by 3).