You don't need the first sentence to solve this problem, but if the GMAT is offering it, take advantage. Each one of the odd integers from 101 to 199 corresponds with one of the first 50 positives odds. 101 is 100 greater than 1, 103 is 100 greater than 3, and so on. So, each of the 50 odds between 101 and 199 is 100 greater than a corresponding term in the series from 1 to 99. That means that the difference between the sum of the first 50 positive odds and the odds from 101 to 199 is: 50(100)=5,000 (There are 50 different numbers in each series, and the difference between each pair of numbers is 100.) If the sum of the first 50 odds is 2,500 and the larger series is 5,000 greater, the sum of the larger series is: 2,500+5,000=7,500, choice (D).