So, if it triples its length every ten minutes and you want to know its length in forty minutes, you multiply one millimeter by three, letting you know its 10 minute point (3 mm). Keep multiplying it by three until you get to the answer you need to! 3 x 3 = 9 (20 min), 9 x 3 = 12 (30 min), 12 x 3 = 36 mm (40 min). Hope I've helped!