x = 1 2 0 ÷ 2 . 7 1 8
Round x to the nearest integer.
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2 . 7 1 8 1 2 0 = 2 7 . 1 8 1 2 0 0 which nearly equals. 2 7 1 2 0 0 = 9 4 0 0 = 4 0 0 × 9 1 = 4 0 0 × 0 . 1 1 1 1 . . . . . = 4 4 . 4 4 4 4 4 . . . . . . .
A N S W E R : [ 4 4 . 4 4 . . . ] = 4 4
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First we approximate the given quotient by e 5 ! = e 1 2 0 < 2 . 7 1 8 1 2 0 < 2 . 7 1 2 0 = 9 4 0 0 = 4 4 . 4 ˉ
Using [ ⋅ ] to denote the nearest integer function and ! n to denote the subfactorial, the above inequality implies 4 4 = ! 5 = [ e 5 ! ] ≤ [ 2 . 7 1 8 1 2 0 ] ≤ [ 4 4 . 4 ˉ ] = 4 4 so the answer is 4 4 .
Notes: Obviously one can just solve this with a piece of paper and pencil [y'know, like before calculators?]. Due to the numbers chosen, however, it's interesting to give a solution which is more combinatorial in nature. Also, you might have noticed I didn't include the nontrivial step of evaluating ! 5 , but anyone interested can evaluate it directly by using the recursive property ! n = n ⋅ ! ( n − 1 ) + ( − 1 ) n ! 1 = 0