The CAS Registry Mumber 5638-41-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 5,6,3 and 8 respectively; the second part has 2 digits, 4 and 1 respectively. Calculate Digit Verification of CAS Registry Number 5638-41: (6*5)+(5*6)+(4*3)+(3*8)+(2*4)+(1*1)=105 105 % 10 = 5 So 5638-41-5 is a valid CAS Registry Number.