Question

11. A boy runs for 10 minutes at a uniform speed of 9 km/h .At what speed should he run for the next 20 minutes so that the average speed comes to 12 km/h? please tell according to class 9

Answer 1

EVEN I AM CLASS 9

Answer:

13.5 km per hour.

Explanation:

Average speed (12km/hr) = Total Distance / Total Time

                        D1 + D2 / T1 + T2

Total Time = 10 min + 20 min = 30 min = 1/2 Hour

D1 = Speed * Time = 9 * 1/6 (10 min = 1/6 hr)

= 3/2 km = 1.5 km

D2 = ?

12 = 1.5 + D2 / 1/2

12 * 1/2 = 1.5 + D2

6 = 1.5 + D2

6 - 1.5 = D2

4.5 km = D2

Speed = Distance / Time

          = 4.5 / 1/3 (20 min = 1/3 hr)

          = 4.5 * 3 (Reciprocal)

          = 13.5 km/hr