How to obtain both values from a voltage attenuator?
So the total resistance of the divider R1 + R2 ||Ri = 1M\$\Omega\$, thus  R2 || 1M\$\Omega\$ = 200K\$\Omega\$ for 0.2 division, so
R1 = 1M\$\Omega\$ - 200K\$\Omega\$ = 800K\$\Omega\$.
R2||1M\$\Omega\$ = 200K\$\Omega\$ so
1/R2 = (1/200K\$\Omega\$) - (1/1M\$\Omega\$) =&gt; R2 = 250K\$\Omega\$