A group of environmental scientists conducted an extensive study on the biodiversity of fish species in two distinct rivers: River A and River B. The table below summarizes the number of different fish species observed during the four sampling seasons: Winter, Spring, Summer, and Fall.

Table: Number of Fish Species Observed

River A:

• Winter: 12
• Spring: 15
• Summer: 20
• Fall: 17

River B:

• Winter: 8
• Spring: 10
• Summer: 13
• Fall: 14

Based on the table, analyze the data and determine the average increase in the number of fish species observed for each river from Winter to Summer.