First question ... what are they intending to do after this sale?  Are they planning on doing it again with another house?

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@The208Accountant wrote:

 Each child put $10,000 toward the purchase  ... the family will renovate and sell the property. 

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If the children are also renovating it, their "investment" is not just money, it is also labor, so I don't see how it could all be called "interest".