Converting units requires that we know the equivalence between them. For example, if we know that 2.54 cm = 1 inch, then it is easy to convert from inches to cm or vice versa.

Remembering that we can always multiply by 1\( \left( \text{in this case with } 1= \frac{ 2.54 \mbox{ cm}}{1 \mbox { in}} \right)\):

\[ 2 \mbox{ in} \times \frac{ 2.54 \mbox{ cm}}{1 \mbox { in}} = 5.08 \mbox{ cm}.\]

Similarly, we can complete more complicated conversions by always multiplying by 1 and cancelling out the units we want to change:

\[ 1 \mbox{ day} \times \frac{ 24 \mbox{ hour}}{\mbox{day}} \times \frac{3600 \mbox{ sec}}{\mbox{hour}} = 24 \times 3600 \mbox{ sec} \]

Thus, there are \( 24 \times 3600 = 86400 \) seconds in a day.

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