Question

The mean lives for a-and b-decay of radioactive substance are 1620 years and 405 years respectively. In how much time will its $\frac{3}{4}th$fraction will decay, if it simultaneously emits a-and b-particles –

• A. 449 years
• B. 399 years
• C. 109 years
• D. 9 years

Answer 1

Answer: (A)

449 years

Answer 2

$\tau = \frac { \tau _ { 1 } \tau _ { 2 } } { \tau _ { 1 } + \tau _ { 2 } } = \frac { 1620 \times 405 } { 2025 } = 324$ yearsT = 0.693

t = 224.5 years

t = 2T = 449 years