Bob Cooper was born in 1900. By 1930 he had 3 sons, all with the Cooper last name. By
1960 each of Bob's 3 boys had exactly 3 sons of their own. By the end of each 30 year time
period, the pattern of each Cooper boy having exactly 3 sons of their own continued. How
many Cooper sons were born in the 30 year period between 1960 and 1990?​