= Solution
For the estimate implied by the supplied <Hubble constant>, use the low-<redshift> <Hubble law> $D\simeq cz/H_0$. Then
$$
D_L\simeq800\,\mathrm{Mpc},\qquad D_S\simeq2000\,\mathrm{Mpc},\qquad D_{LS}\simeq1200\,\mathrm{Mpc}.
$$
With $\theta_E=\pi/10800$ radians, inversion of the <Einstein angular radius> relation gives
$$
\boxed{M_E=\frac{c^2\theta_E^2}{4G}\frac{D_LD_S}{D_{LS}}\simeq5.89\times10^{14}M_\odot.}
$$
The corresponding projected <Einstein radius> is $R_E=D_L\theta_E\simeq233\,\mathrm{kpc}$.
This is the projected enclosed <mass> of the <galaxy cluster>, including all matter, not just its stars. Strictly, the <gravitational lensing> formula uses <angular diameter distances>, and at $z=0.5$ the low-<redshift> approximation is only an estimate. An exact value requires a specified cosmological expansion history and a consistent lens-to-source <angular diameter distance>; it cannot be determined from $H_0$ alone. The later estimates use the same distance approximation consistently.
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