The CAS Registry Mumber 6542-18-3 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,5,4 and 2 respectively; the second part has 2 digits, 1 and 8 respectively. Calculate Digit Verification of CAS Registry Number 6542-18: (6*6)+(5*5)+(4*4)+(3*2)+(2*1)+(1*8)=93 93 % 10 = 3 So 6542-18-3 is a valid CAS Registry Number.