Final answer:
By the late 1960s, Communists controlled a significant portion of the globe, with one-third of the world's population living under communist rule. This influence was primarily due to the extensive reach of the USSR and China.
Step-by-step explanation:
By the late 1960s, it was estimated that one-third of the world's population lived under Communist rule, primarily due to the influence and reach of the Union of Soviet Socialist Republics (USSR) and the People's Republic of China. Significant regions of Eurasia were under the control or influence of these Communist powers. This does not mean that exactly one-third of the world's landmass was communist, but rather that the population living in communist countries constituted about a third of the globe's population.
The Soviet Union and China exerted their influence not only in their immediate regions but also in countries like Vietnam and North Korea. They also had effects on countries such as India, non-aligned but often sympathetic to the Soviet Bloc, and in places where revolutions occurred, such as Cuba. The extent of Soviet control included Eastern Europe, with Yugoslavia being a notable exception, and its influence spanned much of the globe through its industrial and military might.
In summary, by 1975, the fraction of the globe controlled by Communists could be represented by the considerable portion of the world's population—about one-third—that lived in countries ruled by Communist parties or under their significant influence. However, the 1980s saw the loosening of this control, and eventually, the dissolution of the Soviet Union in 1991.