Make up of Earth's atmosphere

Explanation

Strategy We will drop the % symbol and write the given number over 100. Then we will simplify the resulting fraction, if possible.

WHY Percent means parts per one hundred, and the word per indicates a ratio (fraction).

Solution

We begin with nitrogen

Drop the % symbol and write 78 over 100

78% = \(\frac{78}{100}\)  

To simplify the fraction, factor 78 as 2 · 39 and 100 as 2 · 50. Then remove the common factor of 2 from the numerator and denominator.

         =\(\require{cancel}\frac{\cancel{\color{red}{2}}^1 \cdot 39}{\cancel{\color{red}{2}}^1 \cdot 50}\)

         = \(\frac{39}{50}\)

Nitrogen makes up \(\frac{78}{100}\) , or \(\frac{39}{59}\) , of Earth’s atmosphere.  Oxygen makes up 21%, or \(\frac{21}{100}\), of Earth's atmosphere.  Other gases make up 1% or \(\frac{1}{100}\) of the atmosphere.