Not too light, not too dark ...

Level pending

At John's home there is a room where there are 7 lamps. When 6 or more lights are on at the same time the room is very clear. John wants to have a party and he wants to know how many different ways exists to illuminate the room since it is not too light or completely dark.


The answer is 119.

Heitor Farias by Heitor Farias , 23, Brazil

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1 solution

Maxima Code 

(%i1) c(n,r) := n!/(r!*(n-r)!)$
(%i2) c(7,1) + c(7,2) + c(7,3) + c(7,4) + c(7,5);
(%o2)                                 119
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