Say that you start from a number
. Then, you subtract
and
from
, which means that the expression is
If we want this expression to be negative, we have
So, in order for the expression to be negative, the two numbers we are subtracting, combined, must be more than the starting number.
For example, if we start from 10 and we subtract 4 and then 8, we have
which is negative, because 4 and 8 combined are more than 10