Answer: it will take 7 hours before until Carolyn catches up with Heather.
Explanation:
By the time Carolyn catches up with Heather, they would have travelled the same distance.
Heather left the mall traveling 16 mph. Let t represent the time that she spent in travelling this distance.
Distance = speed × time
Distance travelled by Heather would be
16 × t = 16t
Then, 3 hours later, Carolyn left traveling the same direction at 28 mph. Time spent by Carolyn in travelling the same distance would be (t - 3) hours
Distance travelled by Carolyn would be
28(t - 3 = 28t - 84
Since the distance travelled is the same, then
16t = 28t - 84
28t - 16t = 84
12t = 84
t = 84/12 = 7