9514 1404 393
Answer:
2:19:45 hours if "average speed" is for all laps
2:48:03 hours if "average speed" is for full-speed laps only
Explanation:
If we assume that the 161 mph average speed is the average over the entire race, then the time is ...
time = distance/speed
time = (2.5 mi/lap)(150 laps)/(161 mi/h) = (375 mi)/(161 mi/h) ≈ 2.3292 h
≈ 2 h 19 min 45 s
__
Additional comment
The meaning of "average speed" in this context is unclear. If we assume the average speed applies only to non-cautionary laps, then the time is ...
time = distance 1/speed 1 + distance 2/speed 2
time = (30 laps)(2.5 mi/lap)/(80 mi/h) +(120 laps)(2.5 mi/lap)/(161 mi/h)
= (75 mi)/(80 mi/h) +(300 mi)/(161 mi/h) ≈ 0.9375 h + 1.8634 h ≈ 2.8009 h
≈ 2 h 48 min 3 s
__
Hours can be converted to HH:MM:SS by subtracting the integer part and multiplying the fractional part by 60, then repeat. The integer parts in this process are the sexagesimal (base-60) digits of HH:MM:SS from left to right.