# Answer to Question #11891 in Electric Circuits for divya sharma

Question #11891

a parallel combination of three resistors takes a current of 7.5A from 30 V supply.if the two resistors are 10 ohm and 12 ohm, find the third one.

Expert's answer

The resistance of the group of 3 resistors is equal to

R = 30 V / 7.5 A =

4 ohm.

Let R3 be the resistance of the third resistor, then

1/R =

1/10 + 1/12 + 1/R3

so,

1/4 = 1/10 + 1/12 + 1/R3

1/R3 = 1/4

- 1/10 - 1/12

= 15/60 - 6/60 - 5/60

= 4/60

whence

R3 = 60/4 = 15 ohm

R = 30 V / 7.5 A =

4 ohm.

Let R3 be the resistance of the third resistor, then

1/R =

1/10 + 1/12 + 1/R3

so,

1/4 = 1/10 + 1/12 + 1/R3

1/R3 = 1/4

- 1/10 - 1/12

= 15/60 - 6/60 - 5/60

= 4/60

whence

R3 = 60/4 = 15 ohm

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## Comments

Vishal Soni07.10.17, 11:26Thanks alot .. for given this difficult answer for question

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