5Ω, 15Ω resistors are connected in series in the circuit. So the resultant resistance of the two = 5 + 15 = 20 Ω.
The above resultant resistance is connected parallel to the 20Ω resistor.
Then the resultant resistance is 1/R = 1/R1 +1/R2 = 1/20 +1/20 = 2/20
1/R = 1/10
R = 10Ω
This above resultant resistance is connected in series to the 10Ω again.
So the total resultant resistance is 10 + 10 = 20Ω.