Answer: 9/49
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Step-by-step explanation:
The fraction 3/7 represents the probability of picking a blue marble since there are 3 blues out of 3+4 = 7 total.
Because the first marble is put back, this means the probability of picking blue does not change. So 3/7 is the probability of picking another blue marble.
Overall, we have (3/7)*(3/7) = 9/49 as the probability of picking two blues in a row.