By direct proportion, drop on 50 Ω resistance = 2.5 × 50/500 = 0.25 V
Drop across CMD or CD = 2.5 + 0.25 = 2.75 V
Drop across 550 Ω resistance = 12 − 2.75 = 9.25 V
I = 9.25/550 = 0.0168 A, I2 = 2.5/500 = 0.005 A
I1 = 0.0168 − 0.005 = 0.0118 A
∴ 0.0118 = 2.75/R; R = 233 Ω