This is an exact duplicate of question 99484 with the same content: 1,6,15,X,45,66,91 and same options. The pattern is differences increasing by 4: 1 to 6 is +5, 6 to 15 is +9, 15 to X is +13 (so X=28), 28 to 45 is +17, 45 to 66 is +21, 66 to 91 is +25. The pattern consistently adds 5,9,13,17,21,25 (arithmetic sequence with common difference 4).