thus Sum (digits (750,10)) = (Sum <%0,5%>) + (Sum <%7%>) by Th1105, AFINSQ_2:55
.= (0 + 5) + (Sum <%7%>) by AFINSQ_2:54
.= 5 + 7 by AFINSQ_2:53
.= 12 ; :: thesis: verum