Heather left the park traveling 12 mph. then, 4 hours later, raquel left traveling the same direction at 24 mph. how long until raquel catches up with heather?

So we know that heather is traveling at 12mph right?
That means she would be 12 multiplied by 4 miles away by the time Raquel left the park right?
So when Raquel leaves the park heather is 48 miles away.
Now in the next hour Raquel travels 24miles and heather travelles 60 miles(48+12).
Next hour Raquel is at 48(24+24)miles and heather at 72(60+12)
Next Raquel is at 72(48+24)miles and Heather at 84(72+12).
Next Raquel,is at 96(72+24) and Heather too is at 96(72+12)
So if we add up the number of hours we get 4 hours.
So and:4 hours

9.8? i'm not really sure