Arithmetic/Numbers Less Than Zero
Although we cannot have less than nothing in real life, we have to assume that since you can go all the way in one direction in counting up, that you can continue to count down in the other direction.
For example, when you're giving somebody apples, you can count those apples up; otherwise, if you take apples away from somebody, you carry out a negative action and, therefore, you should apply the negative counting. This way, such numbers are exactly called negatives, and they represent values less than zero.
If you have given somebody an apple, two apples, or three apples, your apple-giving may be assessed with the numbers 1, 2, or 3, respectively. But if you have taken 2, 4, or even 5 apples away from somebody, the actual "apple-giving" will deserve, indeed, negative assessment and, this way, may be represented by negative numbers, which are, in our case, -2, -4, and -5 respectively.
-3 (pronounced "Negative Three" or "Minus Three)
5 - 7 = -2
10 - 19 = -9