We will have the following:
First, we find the equivalent of 15 & 9 ohms, that is:
[tex]\frac{1}{R}=\frac{1}{9}+\frac{1}{15}\Rightarrow\frac{1}{\text{R}}=\frac{8}{45}\Rightarrow R=\frac{45}{8}[/tex]
Then, we find the equivalent of R and 8 ohms:
[tex]R_1=8+\frac{45}{8}\Rightarrow R_1=\frac{109}{8}[/tex]
The, we find the equivalent of R1 and 25 ohms:
[tex]\frac{1}{R_2}=\frac{1}{(109/8)}+\frac{1}{25}\Rightarrow\frac{1}{R_2}=\frac{309}{2725}\Rightarrow R_2=\frac{2725}{309}[/tex]
Finally, we calculate the equivalent of R2 and 10 ohms, that is:
[tex]R_3=10+\frac{2725}{309}\Rightarrow R_3=\frac{5815}{309}\Rightarrow R_3\approx18.8\Omega[/tex]
So, the equivalent resistance is approximately 18.8 ohms.
