Puzzle of the Week #152 - Resistance is Futile

To combine resistors in series, you add their resistances together. For example 10 ohms and 20 ohms combined in series gives 30 ohms.

To combine resistors in parallel, you take the reciprocal of the sum of reciprocals of the individual resistors. For example 12 ohms and 24 ohms combined in parallel gives you 8 ohms (1/12 + 1/24 = 1/8).

You have 5 resistors, each with a rating of 120 ohms. Combining them in different ways it is possible to achieve a variety of resistances, for example, the arrangement below results in a 100 ohm resistance (60+40).

5 resistors 100.JPG

One of the following values is NOT possible using the 5 x 120 ohm resistors, which is it?

45 ohms

96 ohms

105 ohms

140 ohms

144 ohms