A car travels at a constant speed of 54 kilometers per hour. How many meters does the car travel in 20 seconds?

Convert the speed to meters per second. There are 1,000 meters in a kilometer and 3,600 seconds in an hour, so $54 \text{ km/h} = \frac{54{,}000}{3{,}600} = 15$ meters per second. In 20 seconds the car travels $15 \times 20 = 300$ meters. A common wrong answer is 1,080, which comes from multiplying $54 \times 20$ without converting units. Another is 1,080,000, from converting kilometers to meters but not hours to seconds. A third is 3,000, from dividing by 360 instead of 3,600. Always track units through the conversion.