Answer:
2 months
Explanation:
The selling price of the sweater = 0.5 x $20 = $30
For every month that the sweater doesn't sell, price is reduced by 20% = 0.2 x $30 = $6
After the first month, price is reduced by $30 - $6 = $24
After the second month, price is reduced by $24 - $6 = $18