The series follows a pattern of alternating operations: 11 - 5 = 6, 6 + 1 = 7, 7 + 5 = 12. Alternatively, looking at the differences: -5, +1, +5. This does not form a standard simple progression, but the provided answer 26 suggests a pattern of (11*0.5)+0.5=6, (6*1)+1=7, (7*1.5)+1.5=12, (12*2)+2=26.