What time??
Algebra
Level
1
The distance between two towns A and B is 110 km.A motorcyclist starts from city A towards B at 8 a.m at a speed of 20 km/hr.Another motorcyclist starts from B towards A at 9 a.m at a speed of 25 km/hr.Find the time when will they cross each other?
11 p.m
12 p.m
12 a.m
11 a.m
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1 solution
Here we use the concept of relative motion. Instead of calculating the time required by both the people let us consider the time take by the first person. He starts at 8 am. travels at 20 km/hr.. this means he has traveled 20 km and this time is 9 am. now consider the relative motion of person A with respect to person B. according to person B .... person A is travelling at (20 + 25) 45 km/hr. (think about it).. therefore the time required by A to travel (110-20) 90 km at 45 km.hr is two hours. therefore they cross paths at (1 hr + 2 hr) 11 am. note:- the person B is always at rest and since we are seeing the motion in a relative frame.. person A has to travel the entire 110 km.