The CAS Registry Mumber 394210-42-5 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 3,9,4,2,1 and 0 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 394210-42: (8*3)+(7*9)+(6*4)+(5*2)+(4*1)+(3*0)+(2*4)+(1*2)=135 135 % 10 = 5 So 394210-42-5 is a valid CAS Registry Number.