Lisa had 4.25 in her purse she brought some school supplies for $2.75 how much money does lisa have left

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

In a sale there is 25% of all prices a chair costs £45 in sale how much was it before the sale

Norder to get a certain shade of blue paint, a mixer must have 5 parts white paint to 3 parts blue. if 4 gallons of paint must be mixed, how many gallons of white paint must be used?