Question 545713
the question is asking for the average speed of the last hour. we know that the average speed for the whole 4 hour trip is 45mph, and the average speed for three of the hours is 40mph. an average is all the numbers added together, then divided by the number of numbers. \

the first 3 hours: 40+40+40 which equals 120, then to find the mean, we divide the sum by3.  to see what the total sum for all 4 hrs we multiply 45(average) by 4. that equals 180. 

next we do 180-120 which equals 60. so the average speed driven for the 4th hour is 60 miles per hour