一辆汽车以 74.5 km/h 的平均速度行驶了 2.25 小时。

1. Use the conversion factors for kilometres and hours.

$75\text{ km/h}=75\times \dfrac{1000\text{ m}}{3600\text{ s}}$

2. Calculate the speed in metres per second.

$75\times \dfrac{1000}{3600}=20.833\ldots$

1. Round the speed and time to 1 significant figure.

$74.5\text{ km/h} \approx 70\text{ km/h}$

$2.25\text{ h} \approx 2\text{ h}$

2. Estimate the distance using distance equals speed multiplied by time.

Estimated distance
$= 70 \times 2 = 140\text{ km}$

1. Explain how the rounding affected the estimate.

In part (b), $74.5\text{ km/h}$ was rounded down to $70\text{ km/h}$ and $2.25\text{ h}$ was rounded down to $2\text{ h}$. Since distance $=$ speed $\times$ time, using smaller values gives a smaller distance than the actual distance.