Need on this geometry question. explain how you did it.

On a piece of paper graph f(x)=5•(0.4)^x

How do you find principle and simple interest

Janie has $3. she earns $1.20 for each chore she does and can do fractions of chores. she wants to earn enough money to buy a cd for $13.50. write an inequality to determine the number of chores, c, janie could do to have enough money to buy the cd.