'R' started his journey from point 'X' towards point 'Y' at

Solution

ATQ,

Distance covered by 'R' in the first three hours = 40 × 3 = 120 km

Relative speed of 'S' with respect to 'R' = 70 − 40 = 30 km/h

Time taken by 'S' to overtake 'R' = 120 ÷ 30 = 4 hours

Distance covered by 'S' in 4 hours = 70 × 4 = 280 km

Time taken by 'T' to cover 280 km = 280 ÷ 105 = 2.67 hours

So, 'T' must have left 2.67 hours before 'S' overtook 'R', i.e., 4 − 2.67 = 1.33 hours after 'S' left

Therefore, M = 3 + 1.33 = 4.33 hours