A cyclist leaves at am and reaches at am. Starting from am, every minute a motor cycle leaves and moves towards . Forty-five such motor cycles reach by am. All motor cycles have the same speed. If the cyclist had doubled his speed, how many motor cycles would have reached by the time the cyclist reached ?
A cyclist leaves at am and reaches at am. Starting from am, every minute a motor cycle leaves and moves towards . Forty-five such motor cycles reach by am. All motor cycles have the same speed. If the cyclist had doubled his speed, how many motor cycles would have reached by the time the cyclist reached ?
Solution
Let's break down what's happening here:
A cyclist travels from A to B in exactly 1 hour (10 am to 11 am)
Starting at 10:01 am, motorcycles leave A every minute
45 motorcycles reach B by 11 am
All motorcycles have the same speed
The key question: If the cyclist doubles his speed, how many motorcycles reach B by the time he arrives?
The motorcycles that reach B by 11 am are those that left at:
10:01 am (first motorcycle)
10:02 am (second motorcycle)
...
10:45 am (45th motorcycle)
Critical insight: The motorcycle that left at 10:45 am reaches B exactly at 11 am.
This means: Travel time for each motorcycle
Since all motorcycles have the same speed, every motorcycle takes exactly 15 minutes to travel from A to B.
If the cyclist doubles his speed, he takes half the original time:
Original time
New time
The faster cyclist reaches B at 10:30 am
For a motorcycle to reach B by 10:30 am, it must complete its 15-minute journey by then.
If a motorcycle leaves at time , it reaches B at time minutes.
For the motorcycle to reach B by 10:30 am:
The motorcycles that qualify are those leaving at:
10:01 am
10:02 am
10:03 am
...
10:15 am
From 10:01 am to 10:15 am = 15 motorcycles
15 motorcycles will reach B by the time the faster cyclist reaches B.
Key Learning: When dealing with relative speed problems, we always identify the time each vehicle takes to complete the journey, then work backwards from the target arrival time to find which vehicles qualify.