Question

18. Rita covers half of her journey at 10 km/h
and the remaining half at 15 km/h. Her
average speed is
(a) 10 km/h (b) 15 km/h (c) 12 km/h (d) 14 km/h​

Answer 1

Answer: option c      12 km/hr

Explanation:

let the total distance = x

1st case

v= 10 km/h

s = x/2

t = s/v

 = (x/2)/10

 =  x/20

2nd case

v = 15 km / hr

s = x/2

t  = s/v

  = (x/2) / 15

  = x/15

average speed (v) = total distance /total time    

                               = (x)/x/20+x/30

                               = (x)/50x/600

                               = 600/50x *  x

                               = 600 / 50

                               = 12 km/hr

hope it helps you    

Answer 2

Answer:

Average speed = $\frac{V1+V2 }{2}$

Explanation:

therefore, the average speed of Rita = $\frac{10+15}{2}$=12.5 km/h so the most appropriate answer is 12km/h