Lets consider the positions of each entity at 10 am.

Ron

10 am is at point P

Harry

10 am is at a distance of 30 miles from point P, (since speed of harry is 30mph & he was at point P @9 am)

Professor Snape

10 am has traveled for 2 hours at a speed of 15 mph, hence he is at 30 miles from point Q.

Now Ron & Harry meet at point Q, hence they together have to cover a distance of 30 miles between them.

Relative speed of Ron & Harry = 40 - 30 = 10 mph,

Hence they will take 3 hours to cover the 30 miles between them & meet each other at point Q.

Therefore they meet at point Q at 1 pm.

@1 pm Professor Snape has traveled for an additional 3 hours at speed 15 mph, hence he has traveled for 5 hours & is at 75 miles from point Q.

Now relative distance between Ron & Professor Snape is 75 miles

The relative speed at which they travel is = 40 - 15 = 25 mph

Ron will meet Professor Snape in 75/25 = 3 hours

In these 3 hours, Ron has traveled a distance of 3*40 = 120 miles

Hence Ron meets Professor Snape at a distance of 120 miles from point Q.

Answer D.

Thanks,

GyM

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