Speed from A to B = m miles/hr

Speed from B to A = 50 + m miles/hr

Distance between A and B = R miles.

Relative Speed = m+(50+m)=50+2m

Time after which they will meet = Total Distance/Relative Speed = R/(50 + 2m) hrs.
[For two bodies with speeds x kmph and y kmph the relative speed = (x +y) when they are coming "towards" each other OR
In other words they bridge the distance of x+y km every hour when travelling from two different directions towards each other] ]

Distance from M when they Pass = Speed * Time = m * R/(50 + 2m).
= mR/(50 + 2m) miles. (OPTION C)