The CAS Registry Mumber 42173-25-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 4,2,1,7 and 3 respectively; the second part has 2 digits, 2 and 5 respectively. Calculate Digit Verification of CAS Registry Number 42173-25: (7*4)+(6*2)+(5*1)+(4*7)+(3*3)+(2*2)+(1*5)=91 91 % 10 = 1 So 42173-25-1 is a valid CAS Registry Number.