Question 1021361
The normal set up is
{{{t/A + t/B = 1}}}
In our case A = 8, the 1 job is now only 3/4 of a job (since Jon did 1/4 of it already), and the time worked together, t, is 10/3 hours...so we have
{{{(10/3)/8 + (10/3)/B = 3/4}}}
We need B.  Multiply everything by 8B and get
{{{(10/3)B + 80/3 = 6B}}}
Now multiply by 3 to clear fractions and get
{{{10B + 80 = 18B}}}
{{{8B = 80}}} and
B = 10 hours