Question

A radioactive material decays by simultaneous emission of two particles with respective half lives 1620 and 810 years. The time (in years) after which one-fourth of the material remains is

• A. 1080
• B. 2430
• C. 3240
• D. 4860

Answer 1

Answer: (A)

1080

Answer 2

* A.P. = $\frac { 1 } { 4 }$ = $\frac { 1 } { 2 ^ { \mathrm { n } } }$ (n = 2)

* t = n T_1/2 = n $\left( \frac { T _ { 1 } \times T _ { 2 } } { T _ { 1 } + T _ { 2 } } \right)$ = 2 × $\frac { 1620 \times 810 } { 1620 + 810 }$