For $k=1, 2, 3$ the box ${ B }{ k }$ contains $k$ red balls and $\left( k+1 \right) $ white balls, let $P\left( { B }{ 1 } \right) =\dfrac { 1 }{ 2 } ,P\left( { B }{ 2 } \right) =1$ and $P\left( { B }{ 3 } \right) =\dfrac { 1 }{ 6 } $. A box is selected at random and a ball is drawn from it. If a red ball is drawn, then the probability that it has come from box ${ B }_{ 2 }$ is