dude this is simple. Talk it out to get the equation. If I have X mol H20 times [Deuterium] (concentration = .0150%), I have X mol Deuterium (in this case .790 mol). Or,

X mol H20 x .000150 = .0790 mol Deuterium

The question asks for an answer in moles, so you never have to touch molar mass or Avogadro's number.