Fixed or Variable?

From the book, Chapter 2. Dev’s deliveries: 300 cost $4,100 and 500 cost $5,500. The extra $1,400 for 200 more deliveries is $7.00 each, so the fixed part is $2,000 a month, and 400 deliveries should cost about $4,800. Two periods are a rough guide: if either was unusual, the split will be off.