MathRevolution wrote:
[GMAT math practice question]
Car A drives from P to Q at a constant rate of 100 km per hour. After car A has driven for 1 hour, train B begins traveling from Q to P at a constant rate of 150 km per hour. If the distance between P and Q is 600 km, then what distance has car A traveled when it meets train B?
A. 200 km
B. 220 km
C. 250 km
D. 270 km
E. 300 km
This problem has a couple of potential traps:
a) Distance between P and Q
\(\neq\) Distance between A and B;
b) A's total distance depends on A's total travel time, including the one hour that A travels alone
1) Find actual distance between A and B
Total D between P and Q = 600 km
Actual D between A and B = 500 km
- A travels alone for 1 hour
- A travels (100 kmh * 1hr)= 100 km
- A travels toward B, shortens the distance
- (600-100) = 500km between them when B starts
2) Find time it takes for A and B to meet
Time=D/R, and D=500. We need rate (speed)
Opposite directions: ADD speeds of A and B
Combined speed = (100 + 150) = 250 kmh
Time to meet=D/R:
\(\frac{500}{250}\)= 2 hours
3) Total distance that A has traveled?
Use A's total
time, then multiply by A's
rate TOTAL time that A travels
Alone: 1 hour
To meet B: 2 hours
Total time A travels: 3 hours
TOTAL distance A travels, RT=D:
(100 kmh * 3 hrs) =
300 kmAnswer E
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