The manager at Screaming Trees has been trying to calculate the portion of the company's overhead expenses that is fixed and the portion that is variable. Over the past twelve months, the number of yards of processed was highest in July, when the total monthly overhead costs totaled $31,000 for 36,000 yards of mulch processed. The lowest number of yards of mulch processed in the last twelve months occurred in October, when total overhead costs were $25,000 for 24,000 yards of mulch processed. What is the fixed portion of the monthly overhead expenses?