Question

The speed of light in a vacuum is 2.99x10^8 m/s. calculate its speed in miles per hour.

Answer 1

You'll never get the correct answer without the correct conversion factor. Note carefully that you have no decimal. It should be 
1 km = 0.6214 miles 
1000 m = 1 km 
60 seconds = 1 minute 
60 minutes = 1 hour. 
2.998E8 m/s x (1 km/1000m) x (0.6214 miles/km) x (60 sec/min) x (60 min/hr) = ?