Find a positive integer which when multiplied by 9 gives an integer
between 1100 and 1200, and when multiplied by 13 gives an integer between 1500 and 1600.

Order the numbers from least to greatest.
Put the lowest number on the left.
$\frac{9}{10},\frac{4}{5},\frac{1}{3},\frac{1}{2}$
Construct the number $-\frac{3}{5}$

$\frac{a}{b}=\frac{c}{d}$ iff $ad=bc$

Set of numbers:
natural numbers, $\mathbb{N}=\{0,1,2,\ldots\}$
integers, $\mathbb{Z}=\{0,1,-1,2,-2,3,-3,\ldots\}$
rationals, $\mathbb{Q}=\{\frac{a}{b}\colon a,b\in\mathbb{Z},b\neq0\}$

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