Here we have the equation for the reaction.
`Ba(OH)_2+2HCl rarr BaCl_2+2H_2O`
`Ba(OH)_2:HCl = 1:2`
Amount of HCl consumed `= 0.245/1000xx27.15 = 0.00665 mol`
So amount of `Ba(OH)_2` reacted `= 1/2xx0.00665 mol`
Molarity means the amount of moles solute in one litre of solution.
If the molarity of `Ba(OH)_2` is x then;
`x/1000xx20 = 1/2xx0.00665`
`x = 0.1663`
So the molarity of the `Ba(OH)_2` solution is `0.1663(mol)/L`