The best way to approach this problem is to pick a value for x first.
For simplicity choose 50 percent.
Thus, if she traveled 50 percent of trip at 40 mph and 50 percent at 60 mph, her average would be 50 mph.
Now fill in 50 for x and see where you get an answer of 5.
C.
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Source: Beat The GMAT — Problem Solving |
Let the total distance be 100
So Distance traveled with 40mph = x
Distance traveled with 60mph = 100-x
Now time to travel x (let t1) = x/40
(because time = Distance/speed)
And time to travel 100 - x (let t2) = (100-x)/60
total time = t1 + t2
Total Distance = 100
Now average speed = total distance/ total time
Avg. Speed = 100 / (t1+t2)
= 100/ [x/40 + (100-x)/60]
take LCM and solve:
= 12000/(x+200)
Answer E
Hope this is clear
Karan
So Distance traveled with 40mph = x
Distance traveled with 60mph = 100-x
Now time to travel x (let t1) = x/40
(because time = Distance/speed)
And time to travel 100 - x (let t2) = (100-x)/60
total time = t1 + t2
Total Distance = 100
Now average speed = total distance/ total time
Avg. Speed = 100 / (t1+t2)
= 100/ [x/40 + (100-x)/60]
take LCM and solve:
= 12000/(x+200)
Answer E
Hope this is clear
Karan
