or...
X
...key is for on your calculator? Well...
That's your gold plated
You know (and love) the ones - "You have one red, one green, one blue and one yellow circle. How many ways can you arrange these 4 circles?"
So you might set out to make a grid or chart, starting with red.
1) ০০০০
2) ০০০০
3) ০০০০
etc., etc., until your crayons and patience are worn out.
Or you could just enter 4 on your calculator and then press the factorial (x!) key (you might have to press shift first, depending on your calculator.) Bang. Done. Work smart, not hard.