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Solution:
The division of units of time is the same as the division of whole numbers.
If another unit is required for the quotient, conversion of the unit must be performed correctly.
Noted that \(1\text{ decade}={\color{red}{10}}\text{ years}\).
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| \(\begin{aligned}\color{red}{4\text{ decades }\quad\quad\quad\quad} \\15\space\overline{)7\space0\text{ decades }\quad5\text{ years}} \\\underline{-\space6\space0}\quad\quad\quad\quad\quad\quad\quad\space\space\space\\1\space0\quad\quad\quad\quad\quad\quad\quad\space\space\space \end{aligned}\) |
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The remainder \(10\text{ decades}\) is converted to \(\color{orange}{100\text{ years}}\).
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| \(\begin{aligned}\color{red}{4\text{ decades }\quad\quad\quad7\text{ years}} \\15\space\overline{)7\space0\text{ decades }\quad\quad\quad5\text{ years}} \\\underline{-\space6\space0}\quad\quad\quad\quad\space\underline{+\space1\space0\space0}\quad\quad\space\space \\1\space0\quad\quad\quad\quad\quad\space1\space0\space5\quad\quad\space\space \\\underline{-\space1\space0\space5}\quad\quad\space\space \\0\quad\quad\space\space \end{aligned}\) |
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| \(\begin{aligned} &\quad\space4\text{ decades }7\text{ years} \\&=(4\times10)\text{ years}+7\text{ years} \\&=40\text{ years}+7\text{ years} \\&={\color{red}{47}}\text{ years}. \end{aligned}\) |
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From the calculation,
\(70\text{ decades }5\text{ years}\div15\)
\(={\color{magenta}{47}}\text{ years}\).
Thus, the answer is option (D).
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