The CAS Registry Mumber 77853-20-4 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,7,8,5 and 3 respectively; the second part has 2 digits, 2 and 0 respectively. Calculate Digit Verification of CAS Registry Number 77853-20: (7*7)+(6*7)+(5*8)+(4*5)+(3*3)+(2*2)+(1*0)=164 164 % 10 = 4 So 77853-20-4 is a valid CAS Registry Number.