Hi guys, please give me some clues into solving this question from a workbook.

Question: There are 50 units in an apartment block. 40 units subscribe to the Straits Times, 20 units subscribe to the Business Times, m units subscribe to both Straits Times and Business Times, and n units subscribe to neither Straits Times nor Business Times.

By drawing Venn diagrams, find

a) the smallest possible value of n,

b) the largest possible value of n,

c) the largest possible value of m,

d) the smallest possible value of m.

Thanks a lot!