Many elements are a mixture of isotopes, which have different mass numbers. The relative atomic mass (Ar) of an element is an average value that takes account of the abundance of the isotopes of the element. It is often not a whole number, and it has no units.
To calculate it from percentage abundances, multiply each isotope's mass number by its percentage abundance, add the results, then divide by 100. Chlorine has two isotopes: about 75% of atoms are chlorine-35 and 25% are chlorine-37. So Ar = (35 × 75 + 37 × 25) ÷ 100 = 3550 ÷ 100 = 35.5.
The answer always lies between the mass numbers of the isotopes, and it is closer to the more abundant isotope. For chlorine, 35.5 is nearer to 35 than to 37, because chlorine-35 is three times as common as chlorine-37.