We will show that the only solutions are 135 and 144.
We have a>0,b>0,c>0 and
9(11a+b)=(a+b+c)(abc−1)
- If a+b+c≡0(mod3) and abc−1≡0(mod3), then a≡b≡c≡1(mod3) and 11a+b≡0(mod3). It follows now that
a+b+c≡0(mod9) or abc−1≡0(mod9)
- If abc−1≡0(mod9)
we have 11a+b=(a+b+c)k, where k is an integer
and it is easy to see that we must have 19.
Now we will deal with the case when a+b+c≡0(mod9) or a+b+c=9l, where l is an integer.
- If l≥2 we have a+b+c≥18,max{a,b,c}≥6 and it is easy to see that abc≥72 and abc(a+b+c)>1000, so the case l≥2 is impossible.
- If l=1 we have
11a+b=abc−1 or 11a+b+1=abc≤(3a+b+c)3=27
So we have only two cases: a=1 or a=2.
- If a=1, we have b+c=8 and 11+b=bc−1 or b+(c−1)=7 and b(c−1)=12 and the solutions are (a,b,c)=(1,3,5) and (a,b,c)=(1,4,4), and the answer is 135 and 144.
- If a=2 we have b(2c−1)=23 and there is no solution for the problem.