Question 1086475
<pre>
                 SPEED     TIME          DISTANCE
Fast Plane       r+10     1680/(r+10)     1680
Slow Plan         r       1680/r          1680
Difference                 4
</pre>

{{{1680/r-1680/(r+10)=4}}}

-

{{{420/r-420/(r+10)=1}}}

{{{420(r+10)-420r=r^2+10r}}}
{{{420r+4200-420r=r^2+10r}}}
{{{4200=r^2+10r}}}
{{{r^2+10r-4200=0}}}


discrim, {{{100+4*4200=16900=10^2*13^2=130^2}}}


{{{r=(-10+130)/2}}}

{{{highlight_green(r=60)}}}



Slower Plane:  {{{1680/60=highlight(28)}}}, hours