A shopkeeper charges sales tax of x% up to Rs, 2000 and above it he charges y%. A customer pays a total tax of Rs. 320, when he purchases the goods worth Rs. 6000 and he pays the total tax of Rs. 680 for the goods worth Rs. 6000 and he pays the total tax of RS. 680 for the goods worth Rs. 12000. The value of (x-y).