Two cars leave the origin; one goes north at 45 mph and the other goes south at 40 mph. after how many hours will they be 350 miles apart?

Recall  your d = rt, distance = rate * time.

so, say car N goes north at 45mph, and car S goes south at 40mph.

after "some time" they'd be apart 350 miles, when that happens, N has been travelling for say "t" hours, and S has been travelling for the same "t" hours, their time has to be the same.

now, we know they'd be 350 miles apart, so if say N has gone "d" miles, then S has gone the slack, or "350 - d".

and we know their rates.



which is about 4 hours and 7 minutes with 3.5 seconds.