Combine mean and standard deviation from different groups

Say I have
N = 10, 20, 15 from 3 studies
Mean = 11.8, 15.3, 8.4 from 3 studies
Standard deviation = 2.4, 3.2, 4.1 from 3 studies

How do I calculate mean and standard deviation combined?

Thank you!

What do you mean by combined? Are you assuming that there is an underlying mean that is the same for all studies and that's what you're trying to get?

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