Let length of CD = x When all four sides of a quadrilateral touch a circle, then the opposite sides of that quadrilateral are equal. Then -AD +BC = AB +CD 15 +21 = 18 + x 18 + x = 36 X = 18cm 
156, 182, 210, 240, 272, ?
9 14 38 129 ? 2705 16260
...What will come in place of the question mark (?) in the following series?
2, 5, 14, 41, 122, 365, ?
3 2 3 6 14 ? 115.5
728, 737, ?, 778, 814, 863
69, 35.5, 39.5, 68.25, 152.5, ?
24, 12, 18, 45, ?, 708.75
96 89 ? 90 94 91
...7 12 33 ? 635 3804
...15, 31, 67, 131, ?, 375
