average atomic mass is the weighted average atomic mass of the isotopes relative to their natural abundance
following formula can be used for average atomic mass of Cl
average atomic mass of Cl -( % of 35-Cl x 35 amu )+ (% of 37-Cl x 37 amu)
assuming percentage abundance of 35-Cl to be x and percentage abundance of 37-Cl is then (100-x)
average atomic mass of Cl - (35 * x%) + (37 x (100-x)%)
35.5 = 0.35x + 37 - 0.37x
0.02x = 1.5
x = 75%
therefore relative abundance of 35-Cl is 75%