Read this and answer the question. If the question is unanswerable, say "unanswerable".

As an important railroad and road junction and production center, Hanover was a major target for strategic bombing during World War II, including the Oil Campaign. Targets included the AFA (Stöcken), the Deurag-Nerag refinery (Misburg), the Continental plants (Vahrenwald and Limmer), the United light metal works (VLW) in Ricklingen and Laatzen (today Hanover fairground), the Hanover/Limmer rubber reclamation plant, the Hanomag factory (Linden) and the tank factory M.N.H. Maschinenfabrik Niedersachsen (Badenstedt). Forced labourers were sometimes used from the Hannover-Misburg subcamp of the Neuengamme concentration camp. Residential areas were also targeted, and more than 6,000 civilians were killed by the Allied bombing raids. More than 90% of the city center was destroyed in a total of 88 bombing raids. After the war, the Aegidienkirche was not rebuilt and its ruins were left as a war memorial.

How many bombing raids destroyed 40% of the city?
unanswerable