Question

a car moves with a speed of 30 km per hour for half an hour ,25 kilometre per hour for 1 hour and 40 kilometres per hour for 2 hours find its average speed.

Answer 1

Dear Student,

When the car moves with a speed of 30 km per hour for half an hour:
$$\begin{gathered} Speed=30Km/hr \\ Time\left(T1\right)=0.5hr \\ Dis\tan cetravelled\left(D1\right)=Speed*Time=30*0.5=15Km \end{gathered}$$

When the car moves with a speed of 25 kilometres per hour for 1 hour:
$$\begin{gathered} Speed=25Km/hr \\ Time\left(T2\right)=1hr \\ Dis\tan cetravelled\left(D2\right)=Speed*Time=25*1=25Km \end{gathered}$$

When the car moves with a speed of 40 kilometres per hour for 2 hours:
$$\begin{gathered} Speed=40Km/hr \\ Time\left(T3\right)=2hr \\ Dis\tan cetravelled\left(D3\right)=Speed*Time=40*2=80Km \end{gathered}$$

Therefore,
$AverageSpeed=\frac{TotalDis\tan ceTravelled}{Totaltimetaken}=\frac{D1+D2+D3}{T1+T2+T3}=\frac{15+25+80}{0.5+1+2}=\frac{120}{3.5}=\frac{240}{7}=34.29Km/hr$


Regards