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# 20 identical marbles of green colour and 20 identical marbles of white colour can be arranged in a row in how many ways so that no two same colour marbles are adjacent?

$\begin{array}{1 1} (20 \!)^ 2 \\ 2.(20 \!)^2 \\ 2\\ 1\end{array}$

$\therefore$ The required no. of arrangements = 1+1=2 ways.