If 10% of a radioactive substance decays in every 5 years, then the percentage of the substance what will decay in 20 years will be:
Correct answer: D. 34.4%
- A. 40%
- B. 50%
- C. 65.6%
- D. 34.4%
Explanation
Given:10% decays every 5 years → 90% remains after each 5 years.Time = 20 years → number of 5-year periods = 20 / 5 = 4.Step 1: Find remaining fraction after 20 yearsRemaining fraction after each period = 0.90.After 4 periods:Remaining = (0.90)⁴= 0.6561 → 65.61% remains.Step 2: Find decayed percentageDecayed % = 100% − 65.61%= 34.39%.
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