What is the value of resistor R in the figure(Figure 1) if ?V=6.5V?

given

 V = 6.5 ohm

i = 100 m A

i = 0.1 A

R₁ = 10 ohm , R₂ = 15 ohm , and other remained resistor is R

according to Ohm's law

 V = i R_eq  

R_eq =  V / i

R_eq = 6.5 / 0.1

R_eq = 65 ohm

here R₁ , R₂ , R are in series

so R_eq = R₁ + R₂ + R

65 = 10 + 15 + R

R = 65 - 25

R = 40 ohm