The constant is 1.5 because you have 0.5 constant for the first sequential scan on elements [0-7], 0.5 constant for the second sequential scan [8-15] and 0.5 constant for adding the last element of the result of first sequential scan to all the elements of the result of the second sequential scan.
The constant is 1.5 because you have 0.5 constant for the first sequential scan on elements [0-7], 0.5 constant for the second sequential scan [8-15] and 0.5 constant for adding the last element of the result of first sequential scan to all the elements of the result of the second sequential scan.