Question 870877
What is the difference in the distance between Grace and Tom?  280-230=50
What is the difference in the RATE of speed?  10 mph [given]
If she drives 50 miles further at a rate of 10mph faster, they drive for five hours (50/10)
If she drives 280 miles in 5 hours, her speed is 280/5, or 56 mph
If he drives 230 miles in 5 hours, his speed is 230/5, or 46 mph
The average is her speed + his speed, divided by 2.  (56+46)/2, or 102/2, or 51 mph.