Permutation: 

When question says either of them, then we have to do addition.

Example: In a class, there are 10 boys and 5 girls. The teacher wants to select either a boy or a girl to do some task. In how many ways, the teacher can select a student.

In this case, we have to do addition

i.e. 10 + 5 = 15 ways

When question says and of them, then we have to do multiplication. 

Example: In a class, there are 10 boys and 5 girls. The teacher wants to select a boy and a girl to do some task. In how many ways, the teacher can select a student.

In this case, we have to do multiplication

i.e. 10*5 = 50 ways

The same thing can happen in the case of combination also.