A water tank is 528 cm long and 42 cm wide. It contains water to depth of 18cm. Four identical spheres are placed in the tank and are fully submerged. If the water level rises by 7cm, calculate the radius of the spheres.

the length of the water tank l = 528 cm
the width of the water tank b = 42 cm
the raised water level h = 7 cm
let the radius of the each sphere be r cm
the volume of 4 spheres = volume of the raised water
$$\begin{gathered} 4*\frac{4}{3}\pi r^3=528*42*7 \\ \frac{16}{3}*\frac{22}{7}*r^3=528*42*7 \\ {r}^3=\frac{528*42*7*3*7}{16*22} \\ {r}^3=\frac{33*21*3*7^2}{11} \\ {r}^3=3^3*7^3 \\ r=3*7=21 cm \end{gathered}$$
thus the radius of the sphere is 21 cm

hope this helps you