Answer;
≈ 16.2 Ohms 
Explanation;
R₂ and R₃ are in series; therefore to get total Resistance, we will use; 
= R₂ + R₃ 
= 26 +20
= 46 Ohms
Then the two resistors (R₂ and R₃) combined and R₁ are in parallel to each other;
Therefore, using the formula for parallel resistors; 1/R₁ + 1/R(₂,₃)
1/Rt =1/25 +1/46 
 1/Rt =(25+46)/(25 ×46)
Rt = (25 ×46)/(25+46
 = 16.197 
 ≈ 16.2 Ohms