Anna's mistake is writing Keisha's guess into the equation. It says 250 less than 2 times the number of candy. Therefore, it should be 2x - 250 and the inequality should be 3x + 100 < 250 - 2x.
Can you find your teacher's mistake?
If you reduce the inequality you get:
x < -350
There is no such thing as negative candy so the question is written wrong.