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Amount left after a time, or time for a given fraction, in radioactive decay

This type of question, asked 5 times: 28 Jun 2022, 29 Jul 2022, 29 Jan 2024, 1 Feb 2024, 24 Jan 2026 S2. All 5 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    28 Jun 2022

    145
    A radioactive element has a half- life of 200 days. The percentage of original activity remaining after 83 days is (Nearest integer) (Given: antilog 0.125=1.3330.125 = 1.333, antilog 0.693=0.693 = 4.93)

  2. Question 2

    29 Jul 2022

    145
    Assuming 1μg1\mu g of trace radioactive element XX with a half life of 30 years is absorbed by a growing tree. The amount of XX remaining in the tree after 100 years is ____×10−1μg\_\_\_\_ \times10^{- 1}\mu g. [Given: ln⁡10=2.303; log⁡2=0.30\ln10 = 2.303;\ \log2 = 0.30 ]

  3. Question 3

    29 Jan 2024

    147
    The half-life of radioisotopic bromine - 82 is 36 hours. The fraction which remains after one day is _____ ×10−2\times10^{- 2}. (Given antilog .2006=1.587.2006 = 1.587 )

  4. Question 4

    1 Feb 2024

    54
    The ratio of  14C 12C\frac{\ ^{14}C}{\ ^{12}C} in a piece of wood is 18\frac{1}{8} part that of atmosphere. If half life of  14C\ ^{14}C is 5730 years, the age of wood sample is.... years.

  5. Question 5

    24 Jan 2026 S2

    47
    The half-life of  65Zn\ ^{65}Zn is 245 days. After x days, 75%75\% of original activity remained. The value of x in days is ____\_\_\_\_ . (Nearest integer) (Given : log⁡3=0.4771\log3 = 0.4771 and log⁡2=0.3010\log2 = 0.3010 )