Will someone please explain this to me? Thanks so much.

The following selected transactions were completed during June between Frank Company and Johnson Company: 1. June 6 Frank Company sold merchandise on account to Johnson Company, $15,000, terms FOB destination, 1/15, n/eom. The cost of the merchandise sold was $12,000. 2. June 6 Frank Company paid transportations costs of$ 100 for delivery of merchandise sold to Johnson Company on June 8. 3. June 12 Johnson Company returned $4,000 of merchandise purchased on account on June 6th from Frank Company. The cost of the merchandise returned was $3,000. 4. June 20 Johnson Company paid Frank Company for the purchase of June 8th, less discount and less return of June 12th.