As an electrical engineer, you have designed a complicated circuit and sent it o

Question

As an electrical engineer, you have designed a complicated circuit and sent it off to be manufactured. When the manufactured circuit boards come back, you realize you made a mistake with one of the resistors. It was supposed to be 70 , but the boards were made with a 180 resistor. You do not want to take apart the board, or have them re-manufactured. A cheap solution is to add a second resistor.
You can achieve this by putting the second resistor...

in seriesin parallel    

...with the first one.
What should the resistance of the second resistor be, in order to make the correct equivalent resistance when combined with the first one?

Explanation / Answer

in order to decrease the resistance we need to conect the resistor in parallel only

here 1/70=1/180+1/R

70=180*R/180+R

70*(180+R)=180R

70*180+70R=180R

110R=70*180

R=114.545 OHM

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