A 20% discount means you pay 80% of the marked price.
Why 80%? Well, 100% - 20% = 80%.
The discounted price is 80% times $2000.
Let D = discounted price
D = 0.80($2000) = $1600
What is profit?
Profit = (discounted price - cost)/cost
Let c = cost
We are looking for c.
($1600 - c)/c = 0.25
Multiply both sides by c.
$1600 - c = 0.25c
$1600 = 1.25·cosc
c = $1600/1.25 = $1280
Answer: $1280