11 . The number of torches sold in a store each week follows a Poisson distribution with mean � , where � � 20 , � � . The probability that 25 torches are sold in a particular week is 0.0555460 .
( a ) Find � .
( b ) Hence , find the probability that
[ 2 ]
( i ) at least 19 torches are sold on a particular week ;
( ii ) exactly 1 torch is sold on a particular day .
( iii ) exactly 1 torch is sold for each of the four consecutive days . [ 6 ]
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