Exams › SSC CGL (Prelims) › General
Three of the four triads below follow the same pair of mathematical rules and form a group. Identify the triad that does NOT fit the group. The rules involve two operations on the first number to generate the second and third numbers.
- 11 : 21 : 121
- 7 : 15 : 49
- 2 : 3 : 4
- 9 : 17 : 81
Correct answer: 7 : 15 : 49
Solution
For triads 11:21:121, 2:3:4, and 9:17:81 the rules are: second = 2×first - 1; third = first^2. Test: 11→2(11)-1=21 ✓, 11^2=121 ✓; 2→2(2)-1=3 ✓, 2^2=4 ✓; 9→2(9)-1=17 ✓, 9^2=81 ✓. For 7: 2(7)-1=13≠15, so the rule breaks. Hence 7:15:49 does not belong.
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