Question 104636
{{{100=70/(1+R)+60/(1+R)^2}}}
{{{10(1+R)^2=7(1+R)+6}}} Multiply both side by {{{(1+R)^2/10}}}.
{{{10(1+2R+R^2)=7(1+R)+6}}} Expand.
{{{10+20R+10R^2=7+7R+6}}} Distribute.
{{{10R^2+20R+10=7R+13}}} Group terms on both sides. 
{{{10R^2+13R-3=0}}} Group terms on one side. 
{{{R= (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
where a=10, b=13, c=-3
{{{R= (-13+- sqrt( 13^2-4*10*(-3) ))/(2*10) }}} 
{{{R= (-13+- sqrt( 169+120))/(20) }}}
{{{R= (-13+- sqrt( 289))/(20) }}}
{{{R= (-13+- 17)/(20) }}}
First solution 
{{{R= (-13+ 17)/(20) }}}
{{{R=1/5}}}
Second solution
{{{R= (-13- 17)/(20) }}}
{{{R= -3/2 }}}
Let's check one solution
{{{100=70/(1+R)+60/(1+R)^2}}}
{{{100=70/(6/5)+60/(36/25)}}}
{{{100=350/6+250/6}}}
{{{100=600/6}}}
{{{100=100}}}
Good answer. 
You can also check the other answer.