There are two fundamental principles - principle of addition and principle of multiplication. These two principles enable to understand permutations and combinations and form the base for permutations and combinations.
If there are two jobs such that one of them can be completed in m ways, and when it has been completed in any one of these m ways, second job can be completed in n ways; then the two jobs in succession can be completed in m × n ways.
If the first job is performed in any one of the m ways, you can associate with this any one of the n ways of performing the second job and thus there are n ways of performing the two jobs without considering more than one way of performing the first. So corresponding to each of the m ways of performing the first job, you have n ways of performing the second job.
Hence, the number of ways in which the two jobs can be performed is m × n.
If there are two jobs such that they can be performed independently in m and n ways respectively, then either of the two jobs can be performed in (m + n) ways.