Question 818811
The distance from the sun to the Andromeda galaxy is 1.2 x 10^19 miles. Light travels at a speed of 5.88 x 10^12 miles per year. How long does it take light to travel from the sun to the Andromeda galaxy. 
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I just need to know whether to multiply or divide.
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Use the units.  You want the answer in time units, years.
miles*(mi/yr) doesn't work
miles/(mi/yr) --> years
Including units helps to avoid doing the wrong operation.