Natural Uranium is a mixture of _ 92 ^ 238 U and _ 92 ^ 235 U with...

Natural Uranium is a mixture of ${ }_{92}^{238} \mathrm{U}$ and ${ }_{92}^{235} \mathrm{U}$ with a relative mass abundance of $140: 1$. The ratio of radioactivity contributed by the two isotopes of natural uranium, if their half-lives are $4.5 \times 10^9$ years and $7.0 \times 10^8$ years respectively is