GATE GG-1 2025 GG Q64: Geological time scale

Th and

Ra are intermediate nuclides in the decay series of

U to

Pb. The half-lives of

Th, and

Ra are 4.47 billion years, 75,000 years, and 1600 years, respectively. At secular equilibrium, when activities are equal, 10 billion atoms of

U are present. The number of atoms of

Ra present at equilibrium is __________. (Answer in integer)

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