For resonance forms of a molecule or ion,

For resonance forms of a molecule or ion, the observed structure is an average of the resonance forms. Sometimes it also depends on how the ion or molecule interact with neighboring molecule or ion. In physics, resonance is described as when a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency. It is not that one structure always corresponds to the observed structure because they will not exist if it is drawn. It is not the same atoms that do not need to be bonded to each other of the resonance structure because it is describing the migration of the atoms not the species itself. 

29.1-12=17.1the answer is this one

It shows that the experiment is designed properly.