= Solution
An inertially confined sphere disassembles on approximately $\tau\sim r/c_s$, while $n_i=\rho/\bar m_i$. Substitute these in the given number-density confinement product to get
$$
\boxed{\rho r\gtrsim10^{20}\bar m_i c_s\equiv A.}
$$
For an equal D-T mix, $\bar m_i\simeq2.5m_u$. Taking $c_s\sim\sqrt{2k_BT/\bar m_i}$ at 10 keV gives $A\simeq0.365$ kg m$^{-2}=0.0365$ g cm$^{-2}$. An adiabatic sound-speed convention changes this by an order-one factor. This estimate implements the \i[supplied] Lawson product and is not the separate, more stringent alpha-stopping areal-density criterion.
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