The ratio of boys to girls is 3:7, which means that for every 3 boys, there are 7 girls.
We can set up an equation to represent this: 3x + 7x = 20, where x is a variable representing the common ratio.
Combining like terms, we have 10x = 20.
Dividing both sides by 10, we find that x = 2.
Therefore, there are 3 * 2 = <<3*2=6>>6 boys in the swimming lesson. Answer: \boxed{6}.
The ratio of boys to girls in a swimming lesson is 3:7. If there are 20 children in total, how many boys are there?
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