Abdul, while driving to school, computes the average speed for his trip to be 20 km/hr. On his return trip along the same route, there is less traffic and the average speed is 40km/hr. What is the average sped for Abdul's trip?

Question

anonymous · Answer

You need to find the total distance and the total time. The total distance is two times the distance to school, 2d.$$s=d/t=>t=d/s$$Where s is speed, d is distance and t is time. So the time taken to get to school is:$$d/s _{1}$$and the time taken to get back is:$$d/s _{2}$$so the total time is:$$d/s _{1}{+}d/s _{2}=d(1/s _{1}+1/s _{2})$$So, as speed = distance/time, divide the total distance by the total time:$$2d/d(1/s _{1}+1/s _{2})=2/(1/s _{1}+1/s _{2})=2/(1/20+1/40)=80/3\approx26.7km/hr$$

anonymous · Answer

@bendt.......I didn't understand the last part of the solution

2(1/20+1/40)=80/3=26.7 
how?
cn u plz explain

anonymous · Answer

Do you mean you don't understand why$$2/(1/20+1/40)=80/3$$or you don't understand how I got that expression?

anonymous · Answer

why?
2(1/20+1/40)=80/3

anonymous · Answer

$$2(1/20+1/40)=2(2/40+1/40)=2/(3/40)=80/3$$