The number of neutrons in a drop of water(20 drops=1ml) at 4 degree C  is

drop of water = 1/20 mL
density of water at 4 °C= 1 g/mL
so mass of 1 drop of water = density x volume =(1 x 1/20)  = 0.05 gm

mass of 1 mole of water = 18 gm
1 mole of water contains = 6.23 x 10²³ molecules of H₂O (Avogadro's number)


1 drop of water contains = (mass of 1 drop /18) moles
                                        = (1/20 x 1/18 ) = 0.00277 moles
So number of molecules in 1 drop of water = 0.00277 x 6.23 x 10²³
                                                                     = 17.3 x 10²⁰ molecules (of H₂O)

1 molecule of H₂O contains = 8 neutons (from oxygen only),
so number of neutrons in 1 drop of water = 8 x (17.3 x 10²⁰ )
                                                                 = 1.38 x 10 ²² neutrons