The CAS Registry Mumber 335-25-1 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 3,3 and 5 respectively; the second part has 2 digits, 2 and 5 respectively. Calculate Digit Verification of CAS Registry Number 335-25: (5*3)+(4*3)+(3*5)+(2*2)+(1*5)=51 51 % 10 = 1 So 335-25-1 is a valid CAS Registry Number.