2001 General Studies, all questions
UPSC Prelims 2001 · General Studies
Question 113: Science and Technology
A radioactive substance has a half-life of four months. Three fourth of the substance would decay in
a3 months
b4 months
c8 months
d12 months
AnswerThe answer is (c).
In one half-life (4 months) half the substance decays, leaving half; in a second half-life (another 4 months) half of that decays, leaving one-fourth. So three-fourths has decayed after two half-lives, that is 8 months. Remember that after n half-lives the fraction left is (1/2)^n.
Why each option is right or wrong
- (a) Three months is less than one half-life, so less than half has decayed.
- (b) After 4 months only half has decayed.
- (c) Right: two half-lives, 8 months.
- (d) After 12 months seven-eighths has decayed.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.