Draw a schematic of a series resistor circuit. Use the symbol of the resistor to draw a resistor on a piece of paper. Consider using an electronic component template to draw it with. (Refer to www.allaboutcircuits.com/vol_1/chpt_2/5.html if you don't know the symbol for a resistor.) Draw the resistor lengthwise on the paper. Label the left end of the resistor symbol "A" and the right end "B." Assign this resistor a value of 20 ohms. Starting at point B, draw another resistor symbol directly to the right of the first resistor. Label the right end of this symbol "C." Assign this resistor a value of 50 ohms.
Calculate the resistance of the series circuit. For the example in step 1, sum the value of the first resistor and the second resistor. The result is 70, since 20 + 50 = 70. This is the resistance of this series resistor circuit. That is to say it is the total resistance between points A and C.
Draw another resistor on the schematic. From point C, draw a third resistor directly to the right of the second resistor. Label the right end of this symbol "D." Assign this resistor a value of 80 ohms.
Calculate the new series resistance. The total new series resistance is the sum of the resistor values between points A and D. For this example, the sum is 150 ohms, since 20 + 50 + 80 = 150.