The CAS Registry Mumber 1781-71-1 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,7,8 and 1 respectively; the second part has 2 digits, 7 and 1 respectively. Calculate Digit Verification of CAS Registry Number 1781-71: (6*1)+(5*7)+(4*8)+(3*1)+(2*7)+(1*1)=91 91 % 10 = 1 So 1781-71-1 is a valid CAS Registry Number.