I'm having problems with rectangle numbers My text book states "A Rectangle number can be shown as a rectangle of dots. A single dot or a line of dots is not regarded as a rectangle. Figure 1.3 shows the rectangle number 12 illustrated in two ways".
So from the statement I came to the conclusion that a rectangle number would be any number that could be arranged in more than one line and was not a square of a prime number; a number unable to be rewritten in a form other than NxN, N^2.
The question asked was "write down the first ten rectangle numbers".
My answers were 6,8,10,12,14,15,16,18,20,21.
When I checked in the back of the book the correct answers are 4,6,8,9,10,12,14,15,16,18.
Where did I go wrong? Please be gentle I'm a newbie.

The definition is equivalent to saying the number must be able to be factored into two numbers both greater than $1$ so that both dimensions of the rectangle are greater than $1$.  These are all the composite numbers including the squares.  For $9$ you can make a $3 \times 3$ array of dots, for example.
