Positive and negative numbers are numbers that have directional value from a given starting point or from zero. Numbers above or to one side, usually right, of zero are designated as positive (+); those below or to the opposite side, usually left, of zero are designated as negative (). Figure 13 is representative of signed numbers on a horizontal scale. 

To add a positive and a negative number, find the difference in their actual values and give this difference the sign (+ or ) of the larger number.
EXAMPLE
The weight of an aircraft is 2,000 pounds. A radio rack weighing 3 pounds and a transceiver weighing 10 pounds are removed from the aircraft. What is the new weight? For weight and balance purposes, all weight removed from an aircraft is given a minus sign, and all weight added is given a plus sign.
FIRST: Add the values for the removed weights.
10 + 3 = 13
NEXT: Prefix the sign for removed weights.
13
THEN: Add the sum of the removed weights to the total weight, following the rule for unlike signs.
+ 2000  13 = + 1987 pounds.
Subtraction
To subtract positive and negative numbers, change the sign of the subtrahend (the number to be subtracted from another) and proceed as in addition.
EXAMPLE
What is the temperature difference between a temperature reading of +20 at 5,000 feet and a reading of 6 at 25,000 feet? Follow the rule, "a change in temperature is equal to the first reading, subtracted from the second reading."
FIRST: Change the sign of the number to be subtracted.
+20 becomes 20.
NEXT: Combine the two terms, following the rule for adding like signs.
(6) + ( 20) = 26 degrees.
Multiplication
The product of two positive numbers is positive (+). The product of two negative numbers is positive (+). The product of a positive and a negative number is negative ().
EXAMPLES
Division
The quotient of two positive numbers is positive. The quotient of two negative numbers is positive. The quotient of a positive and negative number is negative.
EXAMPLES