The cost of a large flat screen tv is decreasing 20% per year.
if a 50 inch flat screen cost $1200, what will it cost in 3 years

Let x be the old price

X= 1200$
The price decreased 20% after 1 year to become price y :
Y= 0.8x
= 0.8 (1200)
= 960$
In the year after y also decreased 20% to be price y’:
Y’= 0.8y
= 0.8 (960)
= 768 $
The price y’ then decreased 20% to become price y’’:
Y’’ = 0.8 y’
= 0.8 (768)
= 614.4 $
Then the price will be 614.4 $ after 3 years