The Division Algorithm – Finding the Quotient and Remainder

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Example 9 What are $q$ and $r$ if $a=75$ and $b=24$? If $a=75$ and $b=25$?


The Division Algorithm

Theorem Let $a\in\mathbb{Z}^{+}\cup\{0\}$ and $b\in\mathbb{Z}^{+}$. Then, there exists distinct integers $q,r\in\mathbb{Z}$ where $r\in[0,b)$ and

$$ a=bq+r. $$

Recall

i.     Least-Integer Principle: A nonempty set of integers that is bounded below contains a smallest element.

ii.    Greatest-Integer Principle: a nonempty set of integers that is bounded above contains a largest element.


Let $a=75$ and $b=24$.

Then, $a=bq+r\Rightarrow75=24q+r$.

We can perform long division to see that $75=24(3)+3$ or simply use inspection.

Or check $\frac{75}{24}=\frac{25}{8}=\frac{24+1}{8}=\frac{24}{8}+\frac{1}{8}=3+\frac{1}{8}$.

Then, $75=24\left(3+\frac{1}{8}\right)=24(3)+\frac{24}{8}=24(3)+3$.

Or simply by inspection.

Therefore, $q=3,r=3$.


The ladder half of the question is clear by inspection, $75=25(3)+0$.

Thus, $q=3,r=0$.

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