When running a 100-meter race Usain Bolt reached his top speed when he was 30 meters from the starting line and had been running for 4.8 seconds since the start of the race. He passed the 70 meter line of the race when he had been running for 7.2 seconds since the start of the race. Let t represent the number of seconds since the start of the race. Let d represent Usain's distance from the starting line of the race. What was the change in time, Δ t , as the number of seconds since the start of the race increased from 4.8 seconds to 7.2 seconds?