The CAS Registry Mumber 3720-88-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,7,2 and 0 respectively; the second part has 2 digits, 8 and 8 respectively. Calculate Digit Verification of CAS Registry Number 3720-88: (6*3)+(5*7)+(4*2)+(3*0)+(2*8)+(1*8)=85 85 % 10 = 5 So 3720-88-5 is a valid CAS Registry Number.