The CAS Registry Mumber 17956-11-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,7,9,5 and 6 respectively; the second part has 2 digits, 1 and 1 respectively. Calculate Digit Verification of CAS Registry Number 17956-11: (7*1)+(6*7)+(5*9)+(4*5)+(3*6)+(2*1)+(1*1)=135 135 % 10 = 5 So 17956-11-5 is a valid CAS Registry Number.