There are 6 boxes numbered 1, 2, ... 6. Each box is to be filled up either with a red or a green ball in such a way that at least 1 box contains a green ball and the boxes containing green balls are consecutively numbered. The total number of ways in which this can be done is:

$\begin{array}{l}6 \text { green - } 1 \text { way } \\ 5 \text { green - } 2 \text { ways } \\ 4 \text { green - } 3 \text { ways } \\ 3 \text { green -} 4 \text { ways } \\ 2 \text { green - } 5 \text { ways } \\ 1 \text { green - } 6 \text { ways } \\ \therefore \text { Total }=21 \text { ways }\end{array}$