Question

1.
A car moves along a straight path. The speedometer of car indicates 20 kmph constantly for
13 minutes and 60 kmph constantly for next 15 minutes. The average speed of the car in 30 minutes is
(1) 20 kmph
2/3 kmph
(3) 40 kmph
3/2 kmph​

Answer 1

Answer:

Required formulas:

Distance = speed * time …. (i)

Average Speed = [total distance] / [total time] ….. (ii)  

Step 1: distance covered during the first 13 minutes

Let distance covered in this case be denoted as “D1”.

Speed = 20 km/hr

Time = 13 minutes = 13/60 hr = 0.216 hr

Using formula (i), we get

∴ D1 = 20 * 0.216 = 4.32 km

Step 2: distance covered during the next 15 minutes

Let distance covered in this case be denoted as “D2”.

Speed = 60 km/hr

Time = 15 minutes = 15/60 hr = 0.25 hr

Using formula (i), we get

∴ D2 = 60 * 0.25 = 15 km

Step 3:

Total distance covered = D1 + D2 = 4.32 + 15 = 19.32 km

Total time given = 30 minutes = 30/60 hr = 0.5 hr

Using formula (ii), we get

Average speed = 19.32 / 30 = 38.64 km/hr

Thus, the average speed of the car in 30 minutes is 38.64 km/hr.