24. 1 / (1/72 - 1/99)

1. Find a common denominator for the fractions in the denominator: $$72 * 99 = 7128$$.
2. Subtract fractions: $$\frac{1}{72} - \frac{1}{99} = \frac{99 - 72}{7128} = \frac{27}{7128}$$.
3. Invert and multiply: $$1 / \frac{27}{7128} = \frac{7128}{27} = \frac{2376}{9}$$.