Posted by Anonymous on Monday, May 23, 2011 at 8:43pm.
To get 40% or more, you cannot have
0, 1, 2, or 3 only correct answers
prob(0 right) = C(10,0) (1/5)^0 (4/5)^10 = .10737
prob(1 right) = C(10,1)(1/5)(4/5)^9 = .26844
prob(2right) = C(10,2)(1/5)^2(4/5)^8 = .30199
prob(3right) = C(10,3)(1/5)3 (4/5)^7 = . 20133
total = .87913
so prob of passing by just guessing = 1 - .87913 = .1209
check my arithmetic, the procedure is correct.
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