Please help with math

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

You open a savings account with a $400 deposit. each month after that, you deposit $25. write an explicit rule to represent the amount of money you deposit into your savings account. how much money will you have in the account on month 12? pleasseee : (

Free points ! becuz of ya'll i got a 100 on my math test. you so much ya'!