How do you write an essay?
A close question are answered only with 'yes' and 'no' while open questions are questions which needs a far but still related answer and uses words such as 'so...' and 'in behalf, we...'
Picking a team of 5 people from a group of 15.
Permutation is a combination of elements in which order matters, and combination is when arrangement doesn't matter.
Order of numbers matter to open the lock
Selecting a president and vice-president
It matters when you choose to arrange them to be president or VP.
Arranging letters of mathematics
In the statement itself, it already says arrangement.
Picking a team of 5 people out of 15
No arrangement involved
Out of the choices, only the last choice involves choosing without having to arrange the elements that is chosen. Thus, the last choice is an example of combination.