The natural abundances of the two stable isotopes of hydrogen (hydrogen and deuterium) are...

The natural abundances of the two stable isotopes of hydrogen (hydrogen and deuterium) are 99.99 percent  and 0.01 percent . Assume that water exists as either H2O or D2O. Calculate the number of D2O molecules in exactly 400 mL of water (density 1.00 g/mL).