Question

Atomic weight of chlorine 35.5 it has 2 isotopes of atomic weight 35&37 .what is the % of heavier isotopes in the sample??

Answer 1

for easy determination take two equation 
 37 X+37 Y =35.5 and
  X+Y=1
        by sub
we got x=0.75 and Y=0.25so as for consideration y is heavier isotope so percentage composition is 25%.

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Answer 2

for easy determination take two equation 
 37 X+37 Y =35.5 and
  X+Y=1
        by sub
we got x=0.75 and Y=0.25so as for consideration y is heavier isotope so percentage composition is 25%.

I hope it will helpful for you