If $\log_8 p = 25$ and $\log_2 q = 5$, then
Aptitude
Logarithm
Difficulty: Medium
Choose an option
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A$p = q^{15}$
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B$p^2 = q^3$
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C$p = q^5$
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D$p^3 = q$
Answer
Correct Answer: $p = q^{15}$
Explanation
Concept & Formula
To relate variables from two different logarithmic equations, convert both to exponential form and express them using a common prime base so their exponents can be directly compared.
$$ \log_a b = x \implies a^x = b $$
Step-by-Step Solution
* **Given:** Two equations: $\log_8 p = 25$ and $\log_2 q = 5$.
* **Calculation:** Convert both equations to exponential form:
Equation 1: $8^{25} = p$
Equation 2: $2^5 = q$
* Notice that the bases $8$ and $2$ are related. Convert the base $8$ in Equation 1 to base $2$:
$$ (2^3)^{25} = p $$
* Simplify the exponents using $(a^m)^n = a^{m \cdot n}$:
$$ 2^{75} = p $$
* Now look at Equation 2 ($2^5 = q$). We need to raise this equation to a power that turns $2^5$ into $2^{75}$ to match $p$.
* Since $75 \div 5 = 15$, raise both sides of Equation 2 to the power of $15$:
$$ (2^5)^{15} = q^{15} $$
$$ 2^{75} = q^{15} $$
* Since both $p$ and $q^{15}$ are equal to $2^{75}$, we can equate them:
$$ p = q^{15} $$
Exam Strategy & Shortcut
Observe the bases: $8$ is $2^3$. This means $p$ grows $3$ times faster in the exponent. $3 \times 25 = 75$. $q$ is at exponent $5$. How many times does $5$ go into $75$? $15$ times. Therefore, $q$ must be raised to the $15$th power to catch up to $p$.
Common Pitfall
A typical mistake is trying to divide the logarithmic equations directly without converting to exponential form or aligning the bases, leading to incorrect algebraic relationships like $p = q^5$.
Final Answer
**Therefore, the correct answer is p = q^{15}.**