Acomputer computer monitor as regular priced at 3: 20 during a2 day sale the price was decreased to 2: 40 what is the percentage decrease

You're looking for the percentage in decimal form in the equation. We'll call it x for right now. So whatever x is it will be multiplied to 3.20 and subtracted from 3.20 to equal 2.40. So, our equation will look like this
3.20-x(3.20)=2.40
subtract both sides of the equation by 3.20 to get
-x (3.20)=2.40-3.20
-x (3.20)=-.80
now divide both sides of the equation by -3.20 to single out x on the left side
x= -.80/-3.20
x=.25
to turn this back into a percentage, we move the decimal to the right 2 places
.25=25%