$(1)$ \[p=1.8+0.102\,\rm{MPa}=1.902\,\rm{MPa}\] $(2)$ \[ 20\,\rm{kgf/cm^2}=20×\frac{9.807}{(0.01)^2} \] \[ p_a=196.1×10^4\,\rm{Pa} \] \[ p=1.96×10^6+0.102×10^6 \] \[ p_a=2.06×10^6 \] \[\therefore p_a=2.06\,\rm{MPa} \] $(3)$ \begin{align}p&= 0.102-\frac{650}{760}×0.1013\\ &= 0.01536\,\rm{MPa} \\ &= 15.4\,\rm{kPa} \end{align}