smith put some money in a saving certificate which earns 8% a year. the yearly interest in $600. how much did smith invest?

Ineed ! the person who answers and shows their work on they got the answer first will be marked the !

The? half-life of a radioactive substance is 20 years. if you start with some amount of this? substance, what fraction will remain in 120 ? years

(60 points) explain this in detail