Percentage change = 100((New Value - Original Value) / Original Value)
So....
100((800 - 1000) / 1000)
= 100(-1/5) = -20%
Considering the question asks what is the percentage 'decrease' and thus has mentioned that the price is decreasing, you should just give the answer as 20%. If it was not mentioned that the value increases or decreases, it would be correct to put it as -20%