Kathy is 60 years of age and self-employed, single taxpayer. During 2019, she reported $100,000 of revenues and $40,000 of expenses relating to her self-employment activities. If Kathy has no other retirement accounts in her name, what is the maximum amount she can contribute to an individual 401(k) for 2019? Assume she paid $8,478 of self-employment tax for 2019.