Problem

Natural uranium is a mixture of three isotopes

^{234}_{92}\text{U}

^{235}_{92}\text{U}

and

^{238}_{92}\text{U}

with mass percentage 0.01%, 0.71% and 99.28% respectively. The half-life of three isotopes are

2.5 \times 10^5 \text{ yr}

7.1 \times 10^8 \text{ yr}

and

4.5 \times 10^9 \text{ yr}

respectively. Determine the share of radioactivity of each isotope into the total activity of the natural uranium.