Answer:
You need to have an average speed of at least 35 miles per hour for the rest of the trip to get to the church on time
Step-by-step explanation:
We use the following relation to solve the question:

In which v is the velocity, d is the distance, and t is the time.
First 15 minutes:
Time of 15 minutes = 0.25 hour, so 
Velocity of 5 miles per hour. So the distance is of:

Final 15 minutes:
The will need to drive 10 - 1.25 = 8.75 miles in 15 minutes = 0.25 hours. So, the average speed will need to be of at least:

You need to have an average speed of at least 35 miles per hour for the rest of the trip to get to the church on time