thus Sum (digits (975,10)) = (Sum <%5,7%>) + (Sum <%9%>) by Th1119, AFINSQ_2:55
.= (5 + 7) + (Sum <%9%>) by AFINSQ_2:54
.= 12 + 9 by AFINSQ_2:53
.= 21 ; :: thesis: verum