Final answer:
To calculate the time it took Kelly to overtake Alex, who was 6 miles ahead, we use their relative speed of 4 mi/h. The result is that it took Kelly 1.5 hours or 90 minutes.
Step-by-step explanation:
We have a classic relative speed problem where Kelly is trying to overtake Alex. Since Alex left the grandparents' house 10 minutes before Kelly, and Alex is traveling at 36 mi/h, he would be 6 miles ahead when Kelly starts (since 10 minutes is 1/6th of an hour and 36/6 equals 6). Kelly is traveling at 40 mi/h, which means she is closing the gap at a speed of 40 - 36 = 4 mi/h. The time it takes Kelly to catch up to Alex can be found by dividing the distance by the relative speed, which equals 6 miles / 4 mi/h = 1.5 hours or 90 minutes.