To; BobKamman

No, her/his response was about the requirement of $27,800, about which I already know, so her/his answer wouldn't help me at all.

What I wasn't sure is why the couple's tax is zero, when their axable income is more than $26K (which is from Capital gain).

And to this question, your answer makes sense, and when I checked the tax calculation, the tax rate is determined to be 0%.