Nanjing is endowed with rich natural resources, which include more than <answer>40</answer> kinds of minerals. Among them, iron and sulfur reserves make up <answer>40</answer> percent of those of Jiangsu province. Its reserves of strontium rank first in East Asia and the South East Asia region. Nanjing also possesses abundant water resources, both from the Yangtze River and groundwater. In addition, it has several natural hot springs such as Tangshan Hot Spring in Jiangning and Tangquan Hot Spring in Pukou.
How many types of minerals are found in Nanjing?
40