Hello Ch.V. Somalingeswara Rao

Let the percentage abundance of Cl-35 be x.

Let the percentage abundance of Cl-37 be 100-x.

So 35(x/100)+37(100-x/100) = 35.5

35x/100+37-37x/100 = 35.5

-2x/100 = 35.5-37

-x = -1.5*100/2

x = 75

So the percentage abundance of Cl-35 is 75%.

and the percentage abundance of Cl-37 is 25%.

So Ratio of Cl-37 and Cl-35 is 25:75 = 1:3

Hope it helps you.