Question 1031005
The general setup is like this
{{{t/A + t/B = 1}}}
where A and B are how long it will take each to do the 1 job separately.
t is how long they work together.
Let A be how long it takes Jessica, then 4A is how long it takes John.  We have
{{{10/A + 10/4A = 1}}}
Multiply by 4A and get
{{{40 + 10 = 4A}}}
{{{50 = 4A}}}
It would take John 50 hours by himself.  (must be some lawn...)