Subtract from 2018

From 2018 2018 , subtract half of it first. Then subtract 1 3 \dfrac{1}{3} rd of the remaining number , then subtract 1 4 \dfrac 14 th of the remaining number and so on until 1 2018 \dfrac{1}{2018} th of the remaining number is subtracted.

What is the remaining number?


The answer is 1.

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2 solutions

Mark Hennings
Apr 9, 2018

Start off with x 1 = 2018 x_1 = 2018 . If we take away half of this, we obtain x 2 = x 1 1 2 x 1 = 2018 2 x_2 \; = \; x_1 - \tfrac12x_1 \; = \; \frac{2018}{2} If we now take away one third of this, we obtain x 3 = x 2 1 3 x 2 = 2018 3 x_3 \; = \; x_2 - \tfrac13x_2 \; =\; \frac{2018}{3} In general, after we have taken away one j j th of the previous number, we are left with x j = x j 1 ( 1 1 j ) = 2018 j x_j \; = \; x_{j-1} \big(1 - \tfrac{1}{j}\big) \; = \; \frac{2018}{j} Thus, at the end, we are left with x 2018 = 1 x_{2018} = 1 , making the answer 1 + 1 = 2 1+1=\boxed{2} .

The sequence 1 / 2 × 2 / 3 × 3 / 4 × n / ( n + 1 ) = 1 / n 1/2 \times 2/3 \times 3/4 \times n/(n+1) = 1/n

Will this be the sequence?

Read the solution posted by mark hennings in the report.

Vilakshan Gupta - 3 years, 2 months ago

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