Translation:In my house there are five mirrors.
That would mean "My house has five mirrors." whereas here we are saying "There are five mirrors in my house." The focus of the sentences are not the same. While basically, you are giving the same information the idea is to reproduce the form used from one language to the other.
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That's because we use the word house for the building and home for the family and all those included in the house. But a mirror is part of the building so does not fit with home. Here is a site explaining the difference a bit better: http://theydiffer.com/difference-between-house-and-home/
Maybe, but the thing is that one could also say: "Στο σπίτι μου πέντε υπάρχουν καθρέφτες". It's a possible construction emphasizing the number of mirrors in the house, but it hasn't been included in the Greek translations. I mean, if we start "shuffling" the word order, for sure some natural (more or less) sentences will come up, but... :p
Greek Duo in general does not accept numerical digits in place of the spelled out words (some answers may be accepted as alternatives but don't count on it); Duo in general does not accept more than one specific answer for listening exercises, i.e. homophones are rejected, alternative spellings are rejected and so on.