Answer: it will take 2.65 hours
Step-by-step explanation:
Let t represent the time it takes for them to be 315 miles apart.
By the time both of them are 315 miles apart, then, they would have travelled a total distance of 315 miles.
Distance = speed × time
Mary left with a speed of 51 mph. This means that the distance that she would travel in t hours is 51 × t = 51t
Alyssa left in the opposite direction at the same time traveling 68 mph. This means that the distance that she would travel in t hours is 68 × t = 68t
Since the total distance is 315 miles, the.
51t + 68t = 315
119t = 315
t = 315/119
t = 2.65 hours
. On the other hand, 
. Then: