Question

1/3rd at the speed of 10mph and the next 1/3rd at the speed of 20mph and next 1/3rd at the speed of 60kmph find average speed

Answer 1

Let total distance be 3x

first 1/3 rd at a speed of 10 mph

so, time = (3x × 1/3)/10 = x/10 h

second 1/3 rd at a speed of 20 mph

so, time = (3x × 1/3)/20 = x/20 h

third 1/3 rd at a speed of 60 mph

so, time = (3x × 1/3)/60 = x/60 h

So, total time = x/10 + x/20 + x/60

= (6x + 3x + x)/60 = 10x/60 = x/6 h

So, average speed = total distance/total time

= 3x / (x/6) = 18 km/h

Answer 2

Let total distance be 3x

first 1/3 rd at a speed of 10 mph

so, time = (3x × 1/3)/10 = x/10 h

second 1/3 rd at a speed of 20 mph

so, time = (3x × 1/3)/20 = x/20 h

third 1/3 rd at a speed of 60 mph

so, time = (3x × 1/3)/60 = x/60 h

So, total time = x/10 + x/20 + x/60

= (6x + 3x + x)/60 = 10x/60 = x/6 h

So, average speed = total distance/total time

= 3x / (x/6) = 18 km/h