SSC CHSL 19 Mar 2020 Shift 3 Paper

Time taken by the car to cover 15 $\mathrm{km}$ distance at speed of $20\mathrm{km}∕\mathrm{h}$
$=\frac{15}{20}=\frac{3}{4}\mathit{h}$
Time taken by the car to cover $20\mathrm{km}$ distance at speed of $30\mathrm{km}∕\mathit{h}$
$=\frac{20}{30}=\frac{2}{3}\mathit{h}$
Time taken by the car to cover $30\mathrm{km}$ distance at speed of $40\mathrm{km}∕\mathrm{h}$
$=\frac{30}{40}\mathrm{h}=\frac{3}{4}\mathrm{h}$
Time taken by the car to cover $12\mathrm{km}$ distance at speed of $30\mathrm{km}∕\mathrm{h}$
$=\frac{12}{30}=\frac{2}{5}\mathit{h}$
Total time taken
$=\frac{3}{4}+\frac{2}{3}+\frac{3}{4}+\frac{2}{5}$
$=\frac{45+40+45+24}{60}$
$=\frac{154}{60}=\frac{77}{30}\mathit{h}$
∴ Average speed of the car for the total journey
$=\frac{\mathrm{Total}\mathrm{distance}}{\mathrm{Total}\mathrm{time}}$
$=\frac{15+20+30+12}{\frac{77}{30}}$
$=\frac{77\times 30}{77}=30\mathrm{km}∕\mathrm{h}$