Let x mL of 0.10M sodium formate be added
No. of moles in xmL of 0.10M sodium formate=0.101000×x
No. of moles in 50mL of 0.05M formic acid=0.051000×50
[Sod.formate][Formic acid]=0.10x2.5=0.04x
Applying the equation
pH=log[Salt][Acid]+pKa
4.0=log0.04x+3.8
or x=39.6mL