There isn't a straight forward answer to this as "A" rated extinguishers are for Carbonaceous material fires and "B" ratings are for flammable liquid fires. It then depends on what is in the extinguisher i.e. if it is foam it will have a reasonably high "A" rating (guessing around 7A), but if it is powder or CO2 t may have little effect on "A" rated fires ans the powder does not cool the fire and the CO2 would dissipate allowing re-ignition. This is why in many premises you will find various types of extinguisher, positioned relative to the items they are for protecting e.g. CO2 for computer rooms, dry powder for kitchens etc. If you use the incorrect extinguisher you can make the fire worse or even kill yourself (water on fat fires or on electrics). To determine the type and use of extinguishers you should be trained in the survey and use before they are needed in anger