in an intercollegiate competition that lasted for 3 days , 115 students took part on day 1 , 80 on day 2 and 100 on day 3 . if 50 took part on day 1 and day 2 and 70 took part on day 2 and day 3 and 40 took part on all three days , how many students took part only on day 3 ?