Income does not necessarily equal expenses.  Maybe Mom spent $50,000 from savings to pay for the daughter's tuition, living expenses, new car, etc.  Maybe the daughter put all of her earned income into a savings account and didn't spend any of it.  You have to ask because you can't assume when it comes to due diligence.

Pub 501 has a support worksheet on page 16:

https://www.irs.gov/pub/irs-pdf/p501.pdf

Rick