Why is D=12 and not 24 when we are doing /20mph? Both trains traveled 12 miles each so the total distance should be 24 right? If no, why so?

Here, Greg assumed the distance to be 12 miles (makes math easier).

Since train x covered this is 1hr, it’s speed is 12mph.

and train y covered it in 1.5hr, it’s speed is 8mph.

Since they move in the same direction, net speed will add up to 20.

Using the formula, distance = speed*time -----> 12 = 20t

t = 12/20 hours

or 36 mins.

Started at 4PM, took 36 minutes to cover, thus meet at 4:36PM.

No, the question asks when do they **meet**.They have to meet somewhere in between, but since there speeds are different, this will not be exactly half way.

If both travel 12 units of distance (adding to 24), they will **surpass** each other (not meet)

\m/

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