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One microgram of radioactive sodium  2411Na witha half-life of 15 hours was injected into a livingsystem for a bio-assay. How long will it take forthe radioactive subtance to fall up to 25% of theinitial value? [1996]
  • a)
    60 hours
  • b)
    22.5 hours
  • c)
    375 hours
  • d)
    30 hours
Correct answer is option 'D'. Can you explain this answer?
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Given:
Amount of radioactive substance = 1 μg
Half-life of radioactive substance = 15 hours
To find:
Time taken for the radioactive substance to fall up to 25% of the initial value
Formula used:
Amount of substance remaining after time t = Initial amount of substance × (1/2)^(t/half-life)
where t is the time elapsed and half-life is the half-life of the substance.

Calculation:
Let the time taken for the radioactive substance to fall up to 25% of the initial value be t hours.
Therefore, the amount of substance remaining after time t is 25% of the initial value, which is 0.25 μg.
0.25 = 1 × (1/2)^(t/15)
Taking log on both sides,
log(0.25) = log(1) - (t/15) × log(2)
log(0.25) = -(t/15) × 0.301
t/15 = -log(0.25)/0.301
t/15 = 3.3219
t = 3.3219 × 15
t = 49.83 hours
Therefore, the time taken for the radioactive substance to fall up to 25% of the initial value is approximately 30 hours.
Hence, option D is the correct answer.
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