How many strings of length 5 can be written using the letters {a, b, c, d, e, f} if no two consecutive letters can be the same? For example, we'd count adede but not acdde.

3750 strings.

Step-by-step explanation:

First we have 6 different letters to choose from (a, b, c, d, e, f) and we will make a string of length 5.

First, we would have to choose one of the letters, to do this we would have 6 choices.

For our second choice, we can only choose from 5 letters since we cannot choose the one that we already chose (no two consecutive letters can be the same).

Then, for our third choice, we would have to choose from 5 different letters (any letter but the one before).

Similarly for our fourth and fifth choice, we can choose 5 different letters.

Then, the total amount of strings would be:

6 x 5 x 5 x 5 x 5 = 6 x 5⁴ = 3750 strings.