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16)
$\frac{1}{ \frac{1}{9} - \frac{1}{12} } = \frac{1}{ \frac{4}{36} - \frac{3}{12} } = \frac{1}{ \frac{1}{12} } = 12$
17)
$\frac{1}{ \frac{1}{5} - \frac{1}{30} } = \frac{1}{ \frac{6}{30} - \frac{1}{30} } = \frac{1}{ \frac{5}{30} } = \frac{1}{ \frac{1}{6} } = 6$
18)
$\frac{1}{ \frac{1}{3} - \frac{1}{4} } = \frac{1}{ \frac{4}{12} - \frac{3}{12} } = \frac{1}{ \frac{1}{12} } = 12$
19)
$\frac{1}{ \frac{1}{10} - \frac{1}{15} } = \frac{1}{ \frac{3}{30} - \frac{2}{30} } = \frac{1}{ \frac{1}{30} } = 30$