解：?$V=\frac {m}{ρ}=\frac { 1125\ \text {g}}{0.9\ \text {g/cm}^3}=1250\ \text {cm}^3$?
?$ S=\frac Vh=\frac {1125\ \text {cm}^3}{5\ \text {cm}}=250\ \text {cm}^2$?
?$ F=pS=3×10^{4}\ \text {Pa}×250×10^{-4}\ \text m^2=750\ \text {N}$?
?$ m=\frac {G}{g}=\frac {750\ \text {N}}{10\ \text {N/kg}}=75\ \text {kg}$?
