Eduardo solved the following inequality, and his work is shown below:

−5(x + 4) + 21 ≥ −3 + 4(x − 8)
−5x − 20 + 21 ≥ −3 + 4x − 32
−5x + 1 ≥ 4x − 35
−9x ≥ −36
x ≥ 4

What mistake did Eduardo make in solving the inequality?

Question

Eduardo solved the following inequality, and his work is shown below:

−5(x + 4) + 21 ≥ −3 + 4(x − 8)
−5x − 20 + 21 ≥ −3 + 4x − 32
−5x + 1 ≥ 4x − 35
−9x ≥ −36
x ≥ 4

What mistake did Eduardo make in solving the inequality?

anonymous · Answer

I cant find it

anonymous · Answer

Eduardo made one simple mistake with the answer and a lot of us do it. xD 
Anyway did you know that when you divide a number by a negative number, the inequality sign changes direction? 
So, this > would become <. 
Or, this ≥ would become ≤. 
Eduardo divided -36 by -9. 9 is negative so we have to change the sign. 
Therefore, his answer should have been: x ≤ 4, as opposed to x ≥ 4. 
Does that make sense? (: