It's a fairly straight forward one i think, but its been a long time since a studied maths!

The problem is...

There are 6 slots in a row.

One tab can be inserted into each slot.

There are 6 different coded tabs (e.g. 1, 2, 3, 4, 5, 6)

Therefore the total number of cominations is 6^6.

However, three or more tabs of the same code cannot lie next to each other.

i.e. the following combinations cannot exist:

2222222

655555

651113

144442

etc..... because there are 3 or more tabs of the same code next to each other.

What is the total number of allowable combinations?