We do not know the house number of the lady or the next door. We only know that after he looked at the next door, he still needed more information. Most of the combinations have unique sums, but two of them 1,6,6 and 2,2,9 add up to 13, so he must have seen a 13, which is why he stated that more information is necessary. When she makes a statement about the 'eldest' that tells him there is one who is older than all of the rest. Other people have given examples arguing for the 1,6,6. There are cases where children could be separated by less than a year and they would have the same whole number but one is older, or if they were born far enough apart that a parent would consider one 'the eldest'. But this is a guessing game and if you have enough information to be 90% sure, you could say the answer with confidence, right? Either he was going to give the right answer and get the sales pitch or walk away without trying. Or there's the rare case which this logic fails, and he guesses wrong.