The CAS Registry Mumber 18637-20-2 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,8,6,3 and 7 respectively; the second part has 2 digits, 2 and 0 respectively. Calculate Digit Verification of CAS Registry Number 18637-20: (7*1)+(6*8)+(5*6)+(4*3)+(3*7)+(2*2)+(1*0)=122 122 % 10 = 2 So 18637-20-2 is a valid CAS Registry Number.