$解：V_水=\frac {m_水}{ρ_水}=\frac {150\ \text {g}}{1\ \text {g/cm}^3}=150\ \text {cm}^3$
$V_石=V-V_水=250\ \text {cm}^3-150\ \text {cm}^3=100\ \text {cm}^3$
$m_石=ρV=2.5\ \text {g/cm}^3×100\ \text {cm}^3=250\ \text {g}$
$需要\frac {250\ \text {g}}{5\ \text {g}}=50個小石子$
