Question

A boy runs for 10 min at a uniform speed of 9 km/
h. At what speed should he run for the next 20 min
so that the average speed comes to 12 km/h?
(1) 13.5 km/h
(2) 10.2 km/h
(3) 8.2 km/h
(4) 7.72 km/h​

Answer 1

$\huge\red{hey....}$

$\huge\mathfrak{Follow \ Me}$

✌✌✌✌✌✌✌

1))13.5 km/h

⤵⤵⤵⤵

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