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It is impossible to give the exact likelihood that either guy is the father. It can be said that it is more likely that guy B is the father because there were more encounters with guy B and because the Plan B would have a greater effect of preventing pregnancy from the encounters on April 7th and 8th, so that the encounters on April 9th and 10th would be more likely to generate a pregnancy, particularly the encounter on April 10th with guy B.
So, it can be said that it is more likely that it is guy B, but a specific likelihood cannot be clearly determined. And if you want certainty as to the father, it would require DNA testing.
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