If you deposit $3000 in a savings account that earns simple interest at the rate of 2.5% per year, how much interest will you have earned after 4 years?

Firstly we need to find out what's 2.5% from $3000:

3000 * 2.5% =
= 3000 * 2.5/100 =
= 3000 * 25/1000 =
= 3 * 25 =
= 75 [dollars]

It means you will earn $75 dollars of interest annualy. After 4 years you will earn:

4 * $75 = $300