Explanation :
For compound gear train
(two or more gear rotate about same axis)
Train value = $\frac{Product\;of\;no.\;of\;teeth\;on\;driving\;gear}{Product\;of\;no.\;of\;teeth\;on\;driven\;gear}$
= $\frac{N_2\cdot N_4\cdot N_5}{N_3\cdot N_5\cdot N_6}=\frac{N_2\cdot N_4}{N_3\cdot N_6}$
$\therefore Gear\;5\;(Teeth\;N_5)\;is\;idle.$