Flip the Sign, Watch the Dot

An inequality is a whole range of answers, and three small things decide which range: the flip when you scale by a negative, the open-or-filled dot at the boundary, and the exact value the question asks for from inside the set. Miss one and the arithmetic is clean while the answer points at the wrong half of the line.

8 STEPS · 6 QUICK CHECKS · ×/÷ by a negative flips the sign · open ≠ filled · v1