Answers

2014-10-16T16:11:17+08:00
The average speed is given as:
v _{average} =  \frac{v_1 + v_2 + v_3 + v_4}{t}
where v average in this problem is 45mph  
         v1 = 40mph
         v2 = 40mph
         v3 = 40mph
          t = 4hours
you are asked to find v4
by substituting the given values you'll have:
45 =  \frac{40+40+40+v_4}{4}
cross multiply 4 and add the values at the right side
180 = 120 + v4
v4 = 180 - 120
v4 = 60mph

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