How long will it take a car to cross a bridge 900 m long if it is travelling at 54 km/h? [closed]

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closed as off-topic by Zev Chonoles, Ross Millikan, Chris Godsil, P.., Stefan HamckeJul 25 '13 at 19:28

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$\ \frac{54 \ \text{kilometers}}{1 \ \text{hour}} = \frac{900 \ \text{meters}}{ \text{?} } \$ –  RecklessReckoner Jul 25 '13 at 19:27
You need to match up the units. A kilometre is $1000$ metres, so $900$ m is $\frac{900}{1000}=0.9$ km. Now use the familiar formula $\text{distance}=\text{rate}\cdot\text{time}$. You know that the distance is $0.9$ km and the rate is $54$ km/h, so just plug these in and solve for the time. –  Brian M. Scott Jul 25 '13 at 19:31
If it's a drawbridge and the middle span is up to let a ship go through, the time involved is not predictable. –  Will Jagy Jul 25 '13 at 20:20

$$54 \frac{km}{hr} = 54 \frac {1000 m}{3600 s} = 15 \frac ms$$.
Now you know the speed in $\frac ms$, and you can use $$s=vt$$