Simplify the following text for a Grade 7 native English-speaking person.
Original: Arch Coal was formed in July 1997 when the privately held Arch Mineral merged with the publicly traded Ashland Coal , Inc. , which had been formed in 1975 as a subsidiary of Ashland Oil .
Simplified: Ashland Coal , Inc. was formed in 1975 as a subsidiary of Ashland Oil . The privately held Arch Mineral Corporation merged with Ashland Coal , Inc. in July 1997 , creating the present-day company .