Electric field by charges

Potential at O = V due to point charge + V due to shell.

$V_{O}=\frac {Q}{4πε(R)}+\frac {Q}{4πε(2R)}$

$V_{P}=V_{shell}+\frac {Q}{4πε(2.5R)}$

$V_{shell}= V_{P}-\frac {Q}{4πε(2.5R)}$

Potential at O = Potential at P as the field inside shell is 0

$V_{shell}= V_{O}-\frac {Q}{4πε(2.5R)}$

$V_{shell}=\frac {Q}{4πε(R)}[1+0.5-0.4]$

$V_{shell}=\frac {11Q}{40πε(R)}$

$\boxed{a=11}\boxed{b=40}$

$\boxed{Ans.= 51}$