Question

An object travels at $50\text{kmph}$ for the first $2$ hours and travels at $70\text{kmph}$ for the next $2$ hours. What is its average speed for the first $4$ hours in knot? $\left[5\text{marks}\right]$

Answer 1

Given:
Speed in first $2$ $\text{h}$ = $50\text{kmph}$
Speed in the next $2$ $\text{h}$ = $70\text{kmph}$

$\left[1.5\text{marks}\right]$

Distance travelled in the first $2$ $\text{h}$
= $50\times 2=100\text{km}$
Distance travelled in the next $2$ $\text{h}$
= $70\times 2=140\text{km}$

$\left[1.5\text{marks}\right]$

$\Rightarrow$Total distance travelled$=100+140=240\text{km}$
Total time taken $=4\text{h}$
$\therefore \text{Average speed}=\frac{\text{Total distance covered}}{\text{Total time taken}}$$=\frac{240}{4}=60\text{kmph}$

$\left[2\text{marks}\right]$

Now,
$1\text{nmi}\approx 1.8\text{km}$
So,
$=\frac{60}{1.8}=33.33\text{knots}$