The CAS Registry Mumber 16510-91-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,6,5,1 and 0 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 16510-91: (7*1)+(6*6)+(5*5)+(4*1)+(3*0)+(2*9)+(1*1)=91 91 % 10 = 1 So 16510-91-1 is a valid CAS Registry Number.