Let the missing number be $x$. $$\begin{aligned} & \frac{21.5}{5}+\frac{21}{6}-\frac{13.5}{15}=\frac{(x)^{\frac{1}{3}}}{4}+\frac{17}{30} \\ & \frac{21.5}{5}+\frac{21}{6}-\frac{13.5}{15}-\frac{17}{30}=\frac{(x)^{\frac{1}{3}}}{4} \end{aligned}$$ LCM of 5, 6, 15 and 30 is 30. $$\begin{aligned} & \frac{129+105-27-17}{30}=\frac{\sqrt[3]{x}}{4} \\ & \sqrt[3]{x}=\frac{190 \times 4}{30}=25.33 \approx 25 \end{aligned}$$ $x=25^{3}$ $x=15625$ Hence, the numbers 15625.
Explanation as extracted from the printed page; notation may be imperfect.