Chlorine has two isotopes, Cl-35 and Cl-37 with the average atomic mass 35.5. what is the percentage composition of Cl-35 and Cl-37 respectively? A 25% and 75%
B 75% and 25%
C 65% and 35%
D 35% and 65%​

75% of Cl-35 and 25% of Cl-37

Explanation:

Equation to find the mass number of an element. repeat the brackets for however many isotopes it has.

You can try guess it because 35.5 is closer to 35 than 37 so it has a higher abundance of Cl-35

thats a little if u want the answer just look it up on google thats what i do haha : )