The CAS Registry Mumber 5681-42-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 5,6,8 and 1 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 5681-42: (6*5)+(5*6)+(4*8)+(3*1)+(2*4)+(1*2)=105 105 % 10 = 5 So 5681-42-5 is a valid CAS Registry Number.