"A truck travels 40 miles from point A to point B in exactly 1 hour...."

That means the truck covers the 40 miles in 1 hour. []

"...When the truck is halfway between point A and point B,..."

That means at the halfway point, the truck spends 30 minutes after leaving A.

"...a car starts from point A and travels at 50 miles per hour...."

That means after the truck spends 30 minutes, the car starts leaving A at 40 miles per hour.

"...How many miles has the car traveled when the truck reaches point B?"

That means when the truck reaches point B, it spents the last 30 minutes of the 1 hour he can drive from A to B.

So the car spents 30 minutes also by that time.

d = rt,

d = (50 mi/hr)(0.5hr) = 25 mi. ------------answer.

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30 minutes is half-hour, is 0.5hr.