Someone me with this problem, I am stuck

[astudentinneed]

here is the question:

A race car driver sets out on a 100-kilometer race. At the quarter-way marker, her pit crew radios that she has averaged only 60 km/hr. How fast must she drive over the remaining distance in order to average 100 km/hr for the entire race?

please help me out, I have been working on this for 2 hrs now, I am stuck?

would I use the average velocity formula? but then I get a screwed up answer like

avg v = 160km/hr / 1.167hr = 137.1 km/hr^2? can someone please help me out, but then what is 137km/hr^2 in standard units?? I got like .0105m/s^2

 

[Doc Al]

Here's a hint: How much total time does she have to complete the race if she is to average 100 km/hr? How much time has she spent on just the first quarter?

 

[esalihm]

try using the definition of average speed which is "total distance/total time"