On a number line, the surface of a river is marked at −230 feet and the top of a building is marked at +25 feet, where 0 represents sea level. what do −230 and +25 represent?

Pls! does anybody know a shortcut for answering these types of questions in the future?

Mat bought a phone for $100. he has to pay $30 per mouth. if he has paid $640 in total, how many mouth has he had the phone?

Onnie is considering doing some long term investing with money that she inherited. she would like to have $50,000 at the end of 30 years. approximately how much money would lonnie need to put into an account earning 6% interest compounded annually in order to meet her goal?