Simply translate the words into equations: (16 - t)v = d (21 - t)w = d (12 - t)(v + w) = d Then, if you substitute v and w in the third line, it turns o

Simply translate the words into equations: (16 - t)v = d (21 - t)w = d (12 - t)(v + w) = d Then, if you substitute v and w in the third line, it turns out that d cancels, and you are left with a quadratic formula of t, one of the two solutions being 6 (and the other one making less sense).