Diphosphorus trioxide + water produces phosphorous acid.

Diphosphorous trioxide is a molecular (covalent) compound. Using the prefixes the molecular formula is ##P_2O_3##

Phosphorous acid is ##H_3PO_3##

##P_2O_3 + H_2O -> H_3PO_3##

To balance this equation we begin be adding a coefficient of 2 in front of the phosphorous acid.

##P_2O_3 + H_2O -> 2H_3PO_3##

We balance the hydrogen by adding a coefficient of 3 in front of the water.

##P_2O_3 + 3H_2O -> 2H_3PO_3##

I hope this was helpful. SMARTERTEACHER