i would neither as five and seven is thirteen, doesn't make grammatical sense. unless it refers to a group; yet this was posted as a maths problem the answer 5 & 7 is 13 is proved incorrect. five and seven are thirteen is also wrong due the word thirteen. however the phrase could refer to a small part of more elaborated information; that is there is too little contain within only the five words in both phrases. thus 5 & 7 are 13 is incorrect. therefore as both have been proven to be incorrect results aligned with the given information i would say the result in: more information needed to accurately assess the question set. primarily are the five and seven both refered to as a group; or are five and seven both refed to as two separate entities.