A parallel combination of three resistors takes a current of 5A from a 20V supply. If the two resistors are of 10 ohm and 8 ohm, find the value of third resistor.

Explanation

V = 20 V
I = 5 A
Let R is net resistance of three resistors having resistances `R_1, R_2 and R_3` in parallel combination.
`R_1 = 10 Omega and R_2 = 8Omega`
` R_3 = ?`
(Net resistance) `R =V/I=20/5=4Omega`
But `1/R=1/R_1+1/R_2+1/R_3`
So `1/4 =1/10+1/8+1/R_3`
`1/R_3=1/4-1/10-1/8`
`1/R_3=(10-4-5)/40=1/40`
`implies R_3 =40Omega`