Answer: Charlie have to drive at a speed of 80miles per hour in order to reach there on time.
Explanation:
At 4pm Charlie realizes he has half an hour to get to his grandmothers house for diner. Knowing that the time he has left is 30 minutes and the distance to his grandmother's place is 40 miles, the only thing that he can control is his speed
Speed = distance travelled / time taken
40/0.5 = 80 miles per hour