You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Nanjing

Context:
Nanjing is endowed with rich natural resources, which include more than 40 kinds of minerals. Among them, iron and sulfur reserves make up 40 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.

Question:
How many types of minerals are found in Nanjing?

40