In the first slot lets calculate the chance of getting a w: 1/5
In the second slot calculate the chance of getting a o: 1/4 (notice how it's 4 not 5 because you already have one letter in the first slot so you have 4 left to choose from.)
In third slot, chance of getting a r: 1/3
In fourth slot, chance of getting a l: 1/2
In the fifth slot, if you got all the first four letters right, than you would be left with one letter, so your chances are 1/1 (100%) for the fifth slot
To calculate probability multiply all the fractions we got above: 1/5 x 1/4 x 1/3 x 1/2 x 1/1
We will then get: 1/(5x4x3x2x1) = 1/120
1/120 is the probability of the 5 slots showing the word "world"
