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Average Speed

A race driver drove around a 9-mile track at 120 mph for 4 miles.
Then at 160 mph for 3 miles.
Then at 200 mph for the last 2 miles.
What is his average speed for the whole 9 miles?

At 120 mph, 120 miles is driven in 3600 seconds.
               =  1 mile is driven in 3600/120 sec
               =  4 miles is driven in 4(3600/120) sec
Similarly,  at 160 mph,  3 miles is driven in 3(3600/160) sec
And,         at 200 mph,  2 miles is driven in 2(3600/200) sec  

Total time to drive 9 miles is 
= 4(3600/120) + 3(3600/160) + 2(3600/200) secs
= 120.0 + 67.5 + 36.0 secs
= 223.5 secs

Then, it takes
 1 sec to travel 9/223.5 miles
 = 3600 sec to travel 3600(9/223.5) miles
 = 3600 sec to travel 144.966 miles
 = 145 miles per hour 

Thus his average speed for the 9 mile course is 145 mph.