Mass = mr x moles
Therefore we rearrange to get
Moles = mass/ mr
Water is H2O so the molecular mass values are
H: 1 , O: 16 (from the periodic table).
We have 2 Hs and 1 O so 1 + 1 + 16 = 18
Mass/ mr = 20/18 = 1.11 moles of water
Hope this helps, feel free to ask if you have any questions :)