The CAS Registry Mumber 15804-66-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,5,8,0 and 4 respectively; the second part has 2 digits, 6 and 6 respectively. Calculate Digit Verification of CAS Registry Number 15804-66: (7*1)+(6*5)+(5*8)+(4*0)+(3*4)+(2*6)+(1*6)=107 107 % 10 = 7 So 15804-66-7 is a valid CAS Registry Number.