90% of the original value is how much it is after every year

1 year: 1000*.90 = 900

2 years: 900*.90 = 810

3 years: 810*.90 = 729

4 years: 728*.90 = 656.10

5 years: 656.10*.90 = 590.49

tehrefore it is worth $590.49 after 5 years

(you could also do this using the equation 1000*(1-.10)5 or 1000(.90)5)