1975 CE probability

In a group of 140 students , 70 are taking mathematics , 50 are taking physics and 30 are taking both subjects .
(i) How many students are taking neither mathematics nor physics?
(ii) How many students are taking either mathematics or physics but not both?
(iii) If a student is picked at random, what is the probability that he is taking physics but not mathematics?
(iv) If, among the students not taking physics, a student is picked at random, what is the probability that he is taking mathematics?

thanks a lot for your help~