In formic acid solution, millimole of COOH2 = Molarity x Volume(ml)
= 0.1 x 500
= 50
So, Millimole of H+ in COOH2 = 2 x 50
In H2SO4 solution, millimole of H2SO4 = 0.05 x 1500
= 75
So, millimole of H+ in H2SO4 = 2 x 75
Total millimole of H+ = (100+150) = 250
Total volume of mixture = 1500+500 = 2000ml.
final concentration of H+ = 250/2000
0.125 M
So, final pH= -log[H+] = -log(0.125)
= 0.90