Could you pls tell me why I have to change this sign. You should look at the sign like an equal sign.
-2/5x -9 < 9/10
-2/5x < 99/10
x < -24.75
Why is this wrong? -
I know the answer should be x> -24.75, but why?
you change the direction of the inequality when multiplying by negative values. This is just a shorthand way of moving terms from one side to the other:
-2/5x - 9 < 9/10
-9 < 9/10 + 2/5x
-9 - 9/10 < 2/5 x
-9.9 < 2/5 x
-9.9 * 5/2 < x
-24.75 < x
This is the same as saying
x > -24.75