Number pattern: find the step (difference) between consecutive terms.

Pattern	=	3.5, ?, 4.5, 5.0, 5.5, ?
Find the difference using known terms: 5.0 - 4.5 = 0.5, 5.5 - 5.0 = 0.5. The pattern adds 0.5 each step.
First missing	=	3.5 + 0.5 = 4.0
Last missing	=	5.5 + 0.5 = 6.0
Check: 4.0 + 0.5 = 4.5. Matches.