RTD Problems with different times

[alexei.cancino]

Hello! I have been having trouble grasping a very specific concept in work problems. Particularly in 2 moving objects starting out or arriving at different times like this one:

https://www.manhattanprep.com/gre/blog/ ... Oe_1G883X4

When you start filling up the RTD chart I always get confused on which train to add or subtract the time. “Ten minutes later, a train leaves Tokyo for Kyoto (Train T) traveling 160 miles per hour”. When I attempted to solve this problem I added 10 minutes to this train rather than subtracted. Can someone shed some light in the matter? Or at least what logic I should follow to tackle this problems? Thanks in advance!

[tommywallach]

Hey Alexei,

There are two ways to take on those questions. The method you're talking about involves algebra, and I don't recommend it. The reason I don't recommend it is EXACTLY your problem: it's hard to remember what to subtract or add from (but, always remember that the entity that leaves LATER is going to be moving for LESS time).

Instead, why not work out what happens in the brief period of time when only ONE entity is moving. Then, you can redo the question as if the two entities are leaving at the SAME time.

Example: Steve and Jen are 200 miles apart. Steve goes 20 mph and Jen goes 30mph. If Steve leaves, heading for Jen, at noon, and Jen leaves, heading for Steve, at 1pm, when do they meet?

Explanation: Let's work out what happens in the first hour (when ONLY Steve is going). He goes 20 miles. So now, Steve and Jen are 180 miles apart. From here, we can pretend they are starting at the same time (1pm). Their combined rate is 50 mph, and they have to cover 180 miles. That will take 3.6 hours.

3.6 hours after 1pm = 4:36.

Does that help?

-t

[alexei.cancino]

Thanks Tommy. That helps a lot!

[tommywallach]

Glad to help!

-t