A straight ramp is constructed with 15 wooden beams placed one after another. The thickness of the first beam is 10cm, and each successive beam is thicker than the previous one by the same amount. If the total thickness of all 15 beams together is 240cm, what is the thickness of the last beam?
(a) 20cm
(b) 23cm
(c) 22cm
(d) 24cm