Since the car loses 15% of it's value each year, it retains 85% of the value.
Value after 1 year = 85% of original value.
Value after 2 years = 85% of value after year 1
Value after 3 years = 85% of value after year 2
and so on...
We could calculate this year by year or we could do a single calculation to obtain the value. We need to take 85% of the original value a total of 5 times, since we're wondering about the value in 5 years.
value after 5 years = 20000(0.85)(0.85)(0.85)(0.85)(0.85)
value after 5 years = 20000(0.85)^5
value after 5 years ≈ 8874.10625
So the value will be 8874.11 pounds in 5 years.