Translate equation into an English sentence and write it out. "Thirteen plus one other number equals fifteen." As in language, find a different way to say it. "What can I add to thirteen to get fifteen?" This way you can also being introducing or continue working on language skills at the same time. Mathematical sentences must be complete sentences, and if anything is missing from it the concept falls apart. So a further question for the children later could be, "What does, 'Thirteen plus one other number' mean?" Draws attention to complete sentences and knowing when they have all the information they need. I've seen this used from early grades through college level math.