MHB What Is the Smallest Integer $a$ for $\frac{1}{640}=\frac{a}{10^b}$?

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To find the smallest integer \( a \) such that \( \frac{1}{640} = \frac{a}{10^b} \), it is necessary to express \( \frac{1}{640} \) in terms of a fraction with a power of ten in the denominator. The solution involves determining the prime factorization of 640, which is \( 2^7 \times 5^1 \). By manipulating the equation, it can be concluded that \( a = \frac{10^b}{640} \) must yield integer values for \( a \) and \( b \). The smallest integer \( a \) that satisfies this equation is found to be 1 when \( b = 7 \). The discussion highlights the importance of understanding fractions and integer relationships in mathematical problem-solving.
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Here is this week's POTW:

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Find the smallest value of $a$ such that $\dfrac{1}{640}=\dfrac{a}{10^b}$, where $a$ and $b$ are integers.

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Congratulations to the following members for their correct solution: (Smile)

1. Ackbach
2. kaliprasad
3. castor28

Solution from Ackbach:
We have
$$a=\frac{10^b}{640}=\frac{10^{b-1}}{64}=\frac{10\cdot 10^{b-2}}{64}=\frac{5\cdot 10^{b-2}}{32}
=\frac{5^2\cdot 10^{b-3}}{16}=\dots=5^6\cdot 10^{b-7}. $$
Hence, for $5^6\cdot 10^{b-7}$ to be an integer, we need $b\ge 7$; the smallest such $b$ is $7$, which makes $a=5^6$.
 
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