I tried to solve this with mason's rule.
but I made only 1 / (1 + 6 + 3).
It must be 1/6 as that Proteus Simulation.
I'm sorry that I cannot upload any pictures about my solution.
It might also help if you were clear as to what you are trying to solve for. Saying it must be 1/6 tells us nothing, especially since 1/6 is not a voltage, a current, or a resistance, but just a number? What must be 1/6?