In a crowd of 50 people in a city 20 of them take the subway when they are off from from work, 25 of them take the bus when they are off from work and 7 of them do not take either of them. If a person is selected randomly from this group, determine the following probabilities. Utilizing a table may be useful.

1) The probability that a randomly selected person takes both the subway and the bus.

2) The probability that a randomly selected person student takes the subway given that they take the bus.

3) The probability that a person uses exactly one of the transportation systems.

4) The probability that a person takes the subway given that they uses exactly one of the transportation systems.