The inequality is still true! If you add a number, say 5 to both sides of the following inequality, does anything change?
3 < 6
3 + 5 < 6 + 5
8 < 11
The inequality is still true. We know the statement holds for subtracting the same number because, in a way, addition and subtraction are pretty much the same operation. If I subtract 5 from both sides, I can think of it like "I add negative 5 to both sides" or something along those lines. It's kind of backwards thinking.
