Question 1172696
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Hi  
The number of combinations of a set of n objects taken r at a time is given by nCr = {{{(n!)/(r!(n - r)!)}}}.   
C(9,4) = 126  the number of the different choices of four classrooms possible
a) C(8,4) = 70 the number of choices the history class <u>cannot</u> be located
b) 126-70 = 56 choices right classroom could be be located
56/126 = 28/63 = 4/9  the fraction of the possible choices that will lead to "success".
Wish You the Best in your Studies.