(e) Suppose that when 450 students are asked which types of public transport they have used in the last month it is found that 220 have travelled by bus, 185 by train and 130 have used the underground. 90 have used both bus and train, 75 have used both underground and train and 70 have used both bus and underground. 20 students have taken all three types of transport. Determine how many students have used none of these. [2]