The half life of radium is about 1600 year. Of 100 gram of radium existing now

The half life of radium is about 1600 year. Of 100 gram of radium existing now, 25g will remain unchanged after

(a) 4800 year
(b) 6400 year
(c) 2400 year
(d) 3200 year

3200 year

100 gram at beginning, I = 0

25 gram remains undecayed.

N / N₀ = (1 / 2)ⁿ ⇒ 25 / 100 = (1 / 2)ⁿ

⇒ 1 / 4 = (1 / 2)ⁿ

Therefore, n = 2

Time = n × T

= 2 × 1600

= 3200 years.

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