Define functions: .
Use those to think about this problem.
You might find Chapter 3 of Part I of Topology and the Language of Mathematics useful. It provides a number of example of problems like yours, with solutions.
A free download of the first 50 pages (which includes Chapter 3 of Part I) is available here:
Bobo Strategy - Topology
Hope it's useful -
For instance, , . Then, taking the constant map, , .
Here, the composition of and is just the first map, \phi. Clearly it is injective but not surjective.