Question

Shabnam takes $20$ minutes to reach her school, if she goes at a speed of $6$ km/h. What should be her speed if she wants to reach school in $24$ minutes?
[3 marks]

Answer 1

Let the speed of Shabnam be $x$ so that she can reach the school in $24$ minutes.

Speed (in km/h)	$6$	$x$
Time (in minutes)	$20$	$24$

To reach the school in more time implies that Shabnam will go with less speed.

[0.5 mark]

Hence, the speed ($S$) and time ($T$) are in inverse proportion.
$\Rightarrow ST=constant$
$\Rightarrow S_1{T}_1=S_2{T}_2$

[1 mark]

$\Rightarrow 6\times 20=x\times 24$
$\Rightarrow x=\frac{6\times 20}{24}=5$ km/h

Therefore, Shabnam will take $24$ minutes to reach school if she goes at a speed of $5$ km/h.

[1.5 marks]