Since we are only asked for the total, T, of the three, we can make the substitution d=b+c, yielding a+d +a*d=13, or d=(13-a)/(a+1). For a=1, d=6, and T=7; and due to the symmetry of a+d+a*d=13, a=6 and d=1 also works, T being 7 in both cases. All the options for a between 1 and 7 yield a d which is a ratio but not an integer. So T=7.