Question : Directions: Select the number from among the given options that can replace the question mark (?) in the following series.
19, 39, 77, 155, ?
Option 1: 288
Option 2: 236
Option 3: 309
Option 4: 311
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Correct Answer: 309
Solution :
Given:
19, 39, 77, 155, ?
Multiply the number by 2 and then add and subtract 1 alternately –
(19 × 2) + 1 = 38 + 1 = 39
(39 × 2) – 1 = 78 – 1 = 77
(77 × 2) + 1 = 154 + 1 = 155
(155 × 2) – 1 = 310 – 1 = 309
Thus, the required missing number in the series is 309. Hence, the third option is correct.
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