# 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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Since you're new, I'd like to give you some advice about the site: To get the best possible answers, you should explain what your thoughts on the problem are. That way, people won't tell you stuff you already know, and they can write answers at an appropriate level; also, people tend to be more willing to help if you show that you've tried the problem yourself. If this is homework, please add the [homework] tag; people will still help, so don't worry. –  Zev Chonoles Jul 25 '13 at 19:24
$\ \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

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

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "Homework questions must seek to understand the concepts being taught, not just demand a solution. For help writing a good homework question, see: How to ask a homework question?." – Zev Chonoles, Ross Millikan, Chris Godsil, P.., Stefan Hamcke
If this question can be reworded to fit the rules in the help center, please edit the question.

Hint: you have two different length units, so convert one to the other. Then you should know distance=rate*time. You have two of the three.

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