Pease explain me how to get the answer?

Condensed monthly operating income data for chestnut Inc. for October is presented below. Additional information regarding chestnut operations is as follows: chestnut allocates common fixed expenses to each store on the basis os sales dollars: Management estimates that closing the town store would result in a ten percent decrease in Mall Store's unit sales. The operating results for October are representative of all months. A decision by Chestnut Inc. to close the town store would result in a monthly increase or decrease in Chestnut's operating income of: $ 4,000 decrease $ 3,000 decrease $ 2,500 decrease $ 1,000 increase $ 4,000 increase Please explain me how to get the