A radioactive elements has a half life of 2500 yrs. In how many years will its mass decay by 90% of its initial mass?

The half life of a radioactive substance is 5xx10^(3) yrs. In how many years will its activity decay to 0.2 times its initial activity? Take log_(10) 5=0.6990 .

The half-life of a radioactive substance is 5000 years. In how many years, its activity will decay to 0.2 times of its initial value ? Given "log"_10 5 = 0.6990.