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JAMB Mathematics 2015 Paper

Question : 16

Total: 61

If log₅20 = x, find x

Solution:

Concept:

Change of base helps evaluate a logarithm whose base is not

10

Explanation:

We are given

\log_5 20 = x

This means

5^x = 20

Take

\log_{10}

of both sides:

\log(5^x) = \log(20)

Using the power rule of logarithms,

x\log 5 = \log 20

Divide both sides by

\log 5

x = \frac{\log 20}{\log 5}

Substitute the values:

x = \frac{1.30103}{0.69897}

Now compute the division:

x \approx 1.861

This matches option C.

Answer:

x \approx 1.861

, so the correct option is C.

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