The CAS Registry Mumber 23740-21-8 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,3,7,4 and 0 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 23740-21: (7*2)+(6*3)+(5*7)+(4*4)+(3*0)+(2*2)+(1*1)=88 88 % 10 = 8 So 23740-21-8 is a valid CAS Registry Number.