Race Car Driver's Average speed

[Pioneer98]

A race car driver must average 200.0 km/h over the course of a time trial lasting ten laps. If the first nine laps were done at 198.0 km/h , what average speed must be maintained for the last lap?

Avg Speed=Distance traveled/Time

218 km/h, I averaged the first 9 laps at 198 and plugged the speed for the tenth lap in until I got an average speed of 200 km/h

 

[BruceW]

I don't think the answer is 218 km/h, but it is close to that. You can work out the answer using algebra. The key equation is Avg Speed=Distance traveled/Time (which you have got written down). You need to think how to write out the 'Time' in the denominator.

 

[HallsofIvy]

In order to have an average speed of at least 200 kmph, the driver must complete all 10 laps, with a length lf r km, so a total distance of 10r km, in 10r/200= (1/20)r hrs.

If he averaged 198 km/hr for the first 9r km then he must have taken 9r/198= r/22 hrs. How much time does he have left for the tenth lap?

 

[Pioneer98]

Okay I got it thanks a lot for the help :)