Answer:
35.48948 u
Step-by-step explanation:
Technically u need to multiply the amu with the percentage of the specific isotope. Because it's a percentage u need to divide by 100. When. You get ur final answers u add the two to get the average atomic mass for the Chlorine.
I hope I helped I suck at explaining
35 x 75.711% / 100 = 26.49885 u
37 x 24.229% / 100 = 8.99063 u
26.49885 + 8.99063 = 35.48948u if u need to round off the nr it will be 35.49 u