thus Sum (digits (375,10)) = (Sum <%5,7%>) + (Sum <%3%>) by Th1091, AFINSQ_2:55
.= (5 + 7) + (Sum <%3%>) by AFINSQ_2:54
.= 12 + 3 by AFINSQ_2:53
.= 15 ; :: thesis: verum