On your birthday you receive a pda for $300. the value of the pda decreases by 20% each year. whatt will its value be 4 years from now?

This can function like a simple interest problem because there's a principal ($300), a rate (20% decrease or -0.2 as a decimal each year), and a time (4 years).
I = Principal x rate x time = (300)(-0.2)(4) = -240In 4 years, the PDA loses $240 worth of value.  So it has a $60 value now (300 - 240).
NOW, that's assuming that the percent decrease is based on the $300 every year.  If the percent decrease is based on the value each year, then it's different.