I'm not really certain about this, but I think that it only lets you enter numbers instead of words when you're translating from Dutch to English, since I suppose that it assumes that you can spell English numbers, but it seems like it want's you to actually spell out the Dutch numbers.
I wrote "Two and four equals six" which sounds a lot more natural to me. Wasn't accepted. Is it just "translate word by word"?
It comes down to the same thing, but you're not including sum anywhere, while you can make a normal English sentence with it. In general it's best to stick close to the original sentence, but you still need to make normal/usual sentences, so often word by word translations don't work.