To answer this you might want to check out the boiling point elevation in solvents
dT = Kb * m
dT = elevation of boiling temperature
Kb = constant of increase in boiling point per molal
m = molal of the solute (in your case the acid)
I'll give you the value of Kb in water: 0.52 oC m-1
First you calculate the molality of your 95% acid, then calculate back the molality of the 20% acid, and finally calculate the new boiling temperature.
PS. Remember, 1 molal = 1 mol soluted in 1000 g solvent