$50\ ml$ of mixture of $NH_{3}$ and $H_{2}$ are decomposed to yield $N_{2}$ and $H_{2}$ and to the resulting mixture $40\ ml\ O_{2}$ is added and the mixture is sparked to yield $H_{2}O$. When the resulting mixture of gases were passed through alkaline pyrogallol $6\ ml$ contraction was observed. Calculate the percentage composition of $NH_{3}$ in the original mixture.