How can I do this estimation problem?

Question

How many water molecules are in a person's body if the person has a mass of 92 kg (about 202 pounds)? Assume the person's body is 61 percent water by weight and that the mass of a water molecule is 18 g/mol.

Answer 1

0.61 * 92 kg = about 56 kg water

18 g/mol can be 18 kg/kmol

dimensional analysis gives us 56 kg / (18 kg/kmol) =3.12 kmols water

then take 1000 times Avagadro's number times 312 moles

1000 * (6.022 * 10^23) * 3.12 = 1.88 *  10^26 water molecules