Explain the problem of double coincidence of wants faced under barter system. How has money solved it? 

Double coincidence of wants implies that the needs of any two individuals should complement each other for the exchange to take place. Money as medium of exchange has removed the double coincidence of wants. Under monetary system money is exchanged for goods and services
A man with the wheat who wants to purchase oil, need not have to find a person having oil and needs wheat. He can sell his wheat in the market for money and then purchase oil with the money thus obtained.