# 7.5.4. Scientific Notation

If sunlight rays travelling at a speed of m/s, determine how long they take to reach a planet that is 126 million kilometres away from the sun.

• A7 minutes
• B21 minutes
• C52 minutes
• D2 minutes

### Example

If sunlight rays travelling at a speed of m/s, determine how long they take to reach a planet that is 126 million kilometres away from the sun.

### Solution

Step 1: Convert 126 million kilometres to metres, and write the distance in standard form.

It is important to use the same length unit (metres) that is used to express the speed of sunlight.

There are metres in 1 kilometre, so multiply the number of kilometres by to convert to metres.

Step 2: Determine how long it takes to travel at a speed of .

Since , we can determine the amount of time by dividing the distance by the speed. Since the speed is expressed as metres per second, the unit of time will be seconds.

Divide the number of seconds by 60 to convert to minutes.

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