A boy runs for 10 minutes at a speed of 18km hr. At what speed should he run for the next 20 minutes so that the average speed becomes 12 km hr

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 = 18* 1/6 (10 min = 1/6 hr)
= 3km
D2 = ?
12 = 3 + D2 / 1/2
12 * 1/2 = 3 + D2
6 = 3 + D2
6 - 3= D2
3 km = D2
Speed = Distance / Time
= 3/ 1/3 (20 min = 1/3 hr)
= 3 * 3 (Reciprocal)
= 9 km/hr