Question

Out of a radioactive sample 90% is found after one day. What percent of the original sample can be found after 2 days ?

• A. 64%
• B. 72%
• C. 81%
• D. 45%

Answer 1

The correct option is C

81%

Equal fractions decay in equal periods of time. Fraction remaining undecayed after one day
$=\frac{90}{100}=0.9$
Fraction remaining undecayed after 2 days $={0.9}^2$
= 0.81