Solving for a Special n n

Algebra Level 3

Solve for n n in the following equation:

2 0 2015 = 2 n × 4 0 9 × 1 0 2000 20^{2015}=2^{n}\times40^9\times10^{2000}

Round n n to the nearest whole number, then what is n + 1 = ? n+1=?


The answer is 2018.

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2 solutions

Chew-Seong Cheong
Sep 12, 2018

2 0 2015 = 2 n × 4 0 9 × 1 0 2000 2 2015 × 1 0 2015 = 2 n + 18 × 1 0 2009 2 n = 2 1997 × 1 0 6 n log 10 2 = 1997 log 10 2 + 6 n = 1997 + 6 log 10 2 1997 + 6 0.3010 2017 n + 1 2018 \begin{aligned} 20^{2015} & = 2^n \times 40^9 \times 10^{2000} \\ 2^{2015} \times 10^{2015} & = 2^{n+18} \times 10^{2009} \\ \implies 2^n & = 2^{1997} \times 10^6 \\ n \log_{10}2 & = 1997 \log_{10} 2 + 6 \\ \implies n & = 1997 + \frac 6{\log_{10} 2} \\ & \approx 1997 + \frac 6{0.3010} \\ & \approx 2017 \\ \implies n+1 & \approx \boxed{2018} \end{aligned}

Thank you for the solution.

Hana Wehbi - 2 years, 9 months ago

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Why did you put the title as "The Year is the Answer?!" That just spoils it for the rest of us because we're given the answer without working it out.

Blan Morrison - 2 years, 9 months ago

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True, but now the question is how to fix this :)

Hana Wehbi - 2 years, 9 months ago

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@Hana Wehbi You can edit the problem title (or probably a moderator would delete the title)

X X - 2 years, 9 months ago

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@X X Ok, l will change it, no problem.

Hana Wehbi - 2 years, 9 months ago

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@Hana Wehbi Thank you!

Blan Morrison - 2 years, 9 months ago

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@Blan Morrison You’re welcome.

Hana Wehbi - 2 years, 9 months ago
B D
Sep 13, 2018

Note: The title has been changed. So this is invalid.

How i soled it without calculating is that the title says that the answer is the year. The year is 2018.

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