"One year is twelve months"
Translation:Mwaka mmoja ni miezi kumi na mbili
Well, it seems that both are used. A Google search for "kumi na wawili" yields 320,000 hits, kumi na miwili 60,000, kumi na viwili 14,000, whereas kumi na mbili gives just over 1 million hits. But the latter would include all the N/N class plurals. So, maybe your native should keep a level head on not detach it to readily :)