Question 720281
Remove the fraction by multiplying by 1-t
so we have
{{{(1-t)*R=(1-t)(r/(1-t))}}}
Simplifying by cancelling on the left hand side gives us
(1-t)R=r
now we divide by R to leave the 1-t on its own, giving
{{{((1-t)R)/R=r/R}}}
simplifying by cancelling on the right hand side gives
{{{1-t=r/R}}}
now take 1 from both sides to give 
{{{1-t-1=(r/R)-1}}}
simplify to get
{{{-t=(r/R)-1}}}
then reverse the signs
{{{t=1-(r/R)}}}