Mary and Kate are running clockwise

BTGModeratorVI wrote: ↑

Sat Jun 13, 2020 5:29 am

Mary and Kate are running clockwise around a circular track with a circumference of 500 meters, each at her own constant speed. Mary runs 1000 meters every five minutes and Kate runs 1000 meters every six minutes. If Mary and Kate start opposite one another on the circular track, how many minutes must Mary run in order to pass Kate and catch her again?

A. 7.5
B. 22.5
C. 750
D. 7.5π
E. 45π

Answer: B
Source: Magoosh

Kudos for a correct solution.[/quote]
To get a better idea of what's happening, let's sketch the set-up:


Mary is faster than Kate, we can say that Kate has a 250 meter head start. In other words, when Mary closes that 250-meter gap, she will pass Kate for the FIRST time.


However, since we want to find the time for Mary to pass Kate for the SECOND time, we can say that Kate has a 750 meter head start.


So we want to determine the time it takes Mary to close the gap from 750 meters to 0 meters.

Mary runs 1000 meters every five minutes and Kate runs 1000 meters every six minutes.
Speed = distance/time

Mary's speed = 1000 meters/5 minutes = 200 meters per minute
Kate's speed = 1000 meters/6 minutes ≈ 167 meters per minute
200 meters per minute - 167 meters per minute = 33 meters per minute

So, the distance gap between Mary and Kate CLOSES at a rate of 33 meters per minute
We want to reduce the gap by 750 meters
Time = distance/rate
So, time = 750/33

STOP!! Before we perform this somewhat tedious calculation, we should scan the answer choices.
When we do so, we see that the answer choices are quite spread apart, which means we can be quite aggressive with our estimation.
So, rather than divide by 33, let's divide by 30

That is, 750/33 ≈ 750/30 = 25
So, the correct answer is close to 25

Answer: B

Cheers,
Brent

BTGModeratorVI wrote: ↑

Sat Jun 13, 2020 5:29 am

Mary and Kate are running clockwise around a circular track with a circumference of 500 meters, each at her own constant speed. Mary runs 1000 meters every five minutes and Kate runs 1000 meters every six minutes. If Mary and Kate start opposite one another on the circular track, how many minutes must Mary run in order to pass Kate and catch her again?

A. 7.5
B. 22.5
C. 750
D. 7.5π
E. 45π

Answer: B

Solution:

Since Mary and Kate start opposite each other, in order for Mary to pass Kate and catch her again, she must run 1½ laps, or 750 meters, more than Kate. Also, since Mary’s rate is 1000/5 = 200 and Kate’s rate is 1000/6 = 500/3, the difference in rates is 200 - 500/3. Thus, the time, in minutes, for Mary to pass Kate and catch her again is

750/(200 - 500/3)

2250/(600 - 500)

2250/100

22.5

Answer: B