The CAS Registry Mumber 71653-65-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,1,6,5 and 3 respectively; the second part has 2 digits, 6 and 5 respectively. Calculate Digit Verification of CAS Registry Number 71653-65: (7*7)+(6*1)+(5*6)+(4*5)+(3*3)+(2*6)+(1*5)=131 131 % 10 = 1 So 71653-65-1 is a valid CAS Registry Number.