Question

You normally drive on the freeway between San Diego and Los Angeles at an average speed of 105km/h , and the trip takes 2h and 20min . On a Friday afternoon, however, heavy traffic slows you down and you drive the same distance at an average speed of only 72.0km/h .

Answer 1

I'm assuming by time taken for usual trip is 2 hrs and 20 minutes, and assuming that all speed values are in km/hr 

For this problem, use this formula: 

average speed = total d travelled / total time taken 

First, you must find how long the distance travelled is. 

average speed = total d travelled / total time taken 
Therefore : d = average speed x total time taken - rearranging 

ie. d = vt 
= 105 km/hr x 2.3333... hrs 
= 245 km 

Now, you must find how long the trip takes on Friday afternoon. 

t = d/v - rearranging equation 
= 245 km / 67 km/h 
= 3.656716418 hrs 
= 3 hrs and 39 minutes (rounded to nearest whole minute) 

Now you must find how much longer the trip on Friday takes. 

time longer = (3 hours and 39 minutes) - (2 hours and 20 minutes) 
= 1 hour and 19 minutes