Inorganic builders are put into detergents to act as a buffer, i.e. to keep the pH close to 7. Sodium tripolyphosphate is a major one used. The equation for its formation is shown below:
aNaH2PO4 + bNa 2HPO4 → cNa 5P3O10 + dH2O
What are the values of a, b, c and d?

Answer:

10) D

The only way to solve this is to try all the options and see if they are correct.