A rectangular tank with a base 20 cm by 15 cm is partly filled with water. A solid metal cube of edge 15 cm is lowered into it and is fully submerged. By how many cm does the water level rise?

A fully submerged solid displaces its own volume, $15^3=3375$ cubic cm. That water spreads over the tank's base of $20\times 15=300$ sq cm, so the rise = 3375/300 = 11.25 cm. The trap 15 assumes the level climbs by the cube's edge, which would need the cube to fill the base exactly; 7.5 halves the edge; 22.5 divides by 150, using only half the base area.