Interchangeable percentages

Algebra Level 2

( 1 0 n ) % ÷ ( n 10 ) % = 10 (10^n)\% \div (n^{10})\% = 10

What is the integer value of n n satisfying the equation above?


The answer is 1.

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1 solution

Rewriting this equation as fractions:

( 1 0 n ) % ÷ ( n 10 ) % = 10 1 0 n 100 ÷ n 10 100 = 10 (10^n)\% \div (n^{10})\% = 10 \Rightarrow \frac{10^n}{100}\div \frac{n^{10}}{100}=10

1 0 n 100 ÷ n 10 100 = 1 0 n 100 100 n 10 = 10 \frac{10^n}{100}\div \frac{n^{10}}{100}=\frac{10^n}{100}\cdot \frac{100}{n^{10}}=10

1 0 n 1 0 2 n 10 1 0 2 = 10 \frac{10^n\cdot 10^2}{n^{10}\cdot 10^2}=10

1 0 n n 10 = 10 \frac{10^n}{n^{10}}=10

1 0 n = 10 n 10 10^n=10\cdot n^{10}

1 0 n 1 = n 10 10^{n-1}=n^{10}

log n 10 = n 1 \log{n^{10}}=n-1

10 log n = n 1 10\cdot\log{n}=n-1

log n = n 1 10 \log{n}=\frac{n-1}{10}

We are looking for a positive integer, which one is n n . If n n is a integer, then n 1 n-1 is a integer, which means that log n \log{n} is rational because n 1 10 \frac {n-1}{10} is rational too. With these restrictions, the only possible numbers for n n is powers of 10. But 10, 100, 1000 or higher, aren't solutions for this equation. Then, 1 is the only number that satisfies these conditions, and log 1 = 0 \log{1}=0 , which works when n 1 10 = 0 \frac{n-1}{10}=0 .

Then the answer is 1 \boxed{1} .

I forget to say that it needs to be rational, and log ( n ) \log (n) need to be rational too. In fact all logarithms in base ten, except the powers of ten is irrational.

Victor Paes Plinio - 4 years, 11 months ago

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Ah, I see your point now, that's a nice way to put it.

Just curious, do you have a proof for your claim

For log 10 n \log_{10} n to be rational, where n n is an integer, n n must be an exponent of base 10 10 , that is n = 1 0 k n=10^k for some integer k k

Hung Woei Neoh - 4 years, 11 months ago

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Thanks for this complement!

Victor Paes Plinio - 4 years, 11 months ago

I find your logic to be flawed. n 1 n-1 is an integer, but that does not mean that n 1 10 \dfrac{n-1}{10} is an integer, so why must log 10 n \log_{10} n be an integer?

Hung Woei Neoh - 4 years, 11 months ago

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