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If 12500 = \(5^{x} \times 2^{y}\), then which of the following can be expressed in the form \(5^{x-1} \times 2^{2y}\) ?

  • A.500
  • B.10000
  • C.2500
  • D.62500

Answer: B

The number is 10000.

Write 12500 in prime factors:

\[ 12500 = 125 \times 100 = 5^{5} \times 2^{2} \]

So \(x = 5\) and \(y = 2\). Then

\[ \begin{aligned} 5^{x - 1} \times 2^{2y} &= 5^{4} \times 2^{4} \\ &= 625 \times 16 = 10000 \end{aligned} \]

Equivalently, \(5^{4} \times 2^{4} = 10^{4}\). 2500 is \(5^{4} \times 2^{2}\), which forgets to double the power of 2, and 62500 is \(5^{6} \times 2^{2}\).

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