Why does the relative atomic mass of chlotine is 35.5 ?

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What is chlotine?
Chlorine has isotopes with mass numbers ranging from 32 g mol−1 to 40 g mol−1. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.

Source:
http://en.wikipedia.org/wiki/Chlorine