First figure out how old Ana is right now by subtracting 3 from 15: 15 - 3 = <<15-3=12>>12
Next, figure out how old Billy is by dividing Ana's age by 2: 12 / 2 = <<6=6>>6
Now figure out how old Mark is by adding 4 to Billy's age: 6 + 4 = <<6+4=10>>10
Now, to figure out Sarah's age, first multiply Mark's age by 3: 10 * 3 = <<10*3=30>>30
Finally, subtract 4 from triple Mark's age to find Sarah's age: 30 - 4 = <<30-4=26>>26
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