posted by M.A. .
Tom and Fred are inseparable twins. They are in a five-person club. How many ways can these five line up for a photo if Tom and Fred must stand together?
treat Tom and Fred as one unit, so there are only 4 to arrange
That would be 4!
But within each those, Tom and Fred could be switched.
So the total number of ways is 2(4!) = 48