venksune wrote:

1. In the 1st hr they would've travelled = 5+10 = 15 miles. So, they are 75 miles apart (90-15 =75) at the end of 1st hour.

2. In the next 1 hour their speeds are 10 +20 = 30 m/hr(relative speed) and hence at the end of the 2nd hour they are 75-30 = 45 miles apart.

3. In the thrid hour their speeds are 20 + 40 = 60m/hr. They would meet each other at 45=60*t => t=3/4 of an hour or 45 minutes.

So, they have met each other end of 2hrs and 45 mins.

i agree with above:

on a different note - if their speeds had remained constant through the journey and we were to find the distance they have travelled when they meet each other:

relative speed = 15mph

distance to cover = 90 miles

so time taken until they meet = 90/15 = 6 hours

so in these 6 hours, Tim would have travelled 60 miles

IF the above is correct - they would meet earlier than 6 hours since now their speeds are doubling by the hour. BUT the distance of 60 miles remains the same - its just covered in less time.

So in the above problem could we use the above logic of 90/15*10 to calc tims distance?

just trying to figure out if my logic is correct.

Thanks!