Question

If 10 % of a radioactive material decays in 5 days, then the amount of the original material left after 20 days is approximately-

• A. 60 %
• B. 65 %
• C. 70 %
• D. 75 %

Answer 1

Answer: (B)

65 %

Answer 2

$\frac { 90 } { 100 }$=

= = $\left( \frac { 1 } { 2 ^ { 5 / \mathrm { T } } } \right) ^ { 4 }$ = $\left( \frac { 9 } { 10 } \right) ^ { 4 }$= 0.65