Question 1168201
<pre>
        mow speed
rider   {{{1/(p-20)}}}
pusher  {{{1/p}}}
</pre>


{{{15/(p-20)+25/p=1}}}-------for the one whole lawn


Solve for p and subtract 20.



Check to be sure you understand how that was set up.
If you do the algebra steps right, you may find an equation 
{{{p^2-60p+500=0}}}
which is factorable as
{{{(p-10)(p-50)=0}}}


The solution which works is {{{highlight(p=50)}}}, the time for the push motor to do the job alone; and then {{{p-20=highlight(30)}}}, the time for the rider mower to do the job alone.