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i know!
9 k5 Q% L/ m* N# V  B5 q0 jwhen they can have 15 mins discussion.5.39.217.770 x# @' L7 B7 m2 Z; F
make 1 of them to do a specific job whcih is counting
7 j1 [" |) h6 \4 O. fonly him can turn on off off the light.! l' |1 ^+ u9 p: _8 G* M! n6 V
Basically, if you are the first time to get in the room, turn on the light, 公仔箱論壇# p* r3 c# i: H# E( P  Z
then wait for "the special man" to turn the light off....
7 x9 h2 Q, Q/ o% p" A1 E: Q公仔箱論壇when "the special man" count in up to 30 (included himself)
9 m1 F9 }, M8 ?$ U$ Tthen it is done!