Answer:
This looks like an addition (summation is the technical term) series. Here we need to figure out the difference between each number in the series. The difference between 6.75 and 9.25 is 2.5 (You can subtract the larger number from the smaller number to find out). So, 6.75-2.5=4.25.
So the correct answer in the blank is 4.25, and the rule is +2.5.
Explanation: