The crowded scene on "Black Friday" at Macy's in Manhattan. The U.S. population is projected to hit 400 million in 2051, Census says, up from 321 million in 2015.
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Indian schoolchildren write English alphabets on slates at a primary school outside Hyderabad in June. India is on track to overtake China as the most populous nation in just 16 years.
Noah Seelam/AFP/Getty Images