Given : 2D + 1L = 100,
And D + L = 70
This is a simultaneous equation can be solved as follows
2D + L = 100
- D - L = -70
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D - 0 = 30
i.e D (number of dudes) = 30
Therefore, since we already know the fraternity sold 70 tickets in total,
70 -30 = 40 (the number of ladies attended)