The CAS Registry Mumber 91978-13-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 9,1,9,7 and 8 respectively; the second part has 2 digits, 1 and 3 respectively. Calculate Digit Verification of CAS Registry Number 91978-13: (7*9)+(6*1)+(5*9)+(4*7)+(3*8)+(2*1)+(1*3)=171 171 % 10 = 1 So 91978-13-1 is a valid CAS Registry Number.