A decade is defined as a complete set of consecutive nonnegative integers that have identical digits in identical places, except for their units digitsUnderstanding the pattern here is key ! Let's try a few examples for decades as per above definition
100, 101,102,103,104,105 ....,109 - units digit differ, all other digits are identical
200, 201,202,203,204,205.....,209
420,421,422,423,424,425......,429
From question stem, we know that the first decade consists the smallest integers that meet the above definition
1st decade - 00,01,02,03,04,05,06,07,08,09
The second decade has the next smallest integers that meet the criteria
2nd decade - 10,11,12,13,14,15,16,17,18,19
We need to find the
decade in which the prime numbers contain the same set of units digits as do the prime numbers in the second decade 2nd decade - 10,
11,12,
13,14,15,16,
17,18,
193rd decade - 20,21 --> 21 is not prime . So stop and let's move to the next decade
4th decade - 30,31,32,33 --> 33 is not prime. Stop !
5th decade - 40,41,42,43,44,45,46,47,48,49 -- > 49 is not prime
6th decade - 50,51 --> 51 is not prime . Stop !
7th decade - 60,61,62,63 --> 63 is not prime. Stop!
8th decade - 70,71,72,73,74,75,76,77 --> 77 is not prime. Stop!
9th decade - 80,81 --> 81 is not prime. Stop!
All options have been eliminated except one. So Answer should be (E)
10th decade - 90,91,92,93 -->93 is not prime. Stop!
11th decade - 100,
101,102,
103,104,105,106,
107,108,
109 -
Match !Therefore, Correct Answer is (E) i.e eleventh decadeP.S . Took me almost 5 min to figure out the decade definition! I will definitely skip such a question in GMAT exam.