Question 518777
Call delivering the papers 1 job
Let {{{ t }}} = Rob's time to deliver the papers in hours
{{{ 4t }}} = younger brother's time 
Add their rates to get rate working together
{{{ 1/t + 1/(4t) = 1/2 }}}
Multiply both sides by {{{4t}}}
{{{ 4 + 1 = 2t }}}
{{{ 2t = 5 }}}
{{{ t = 5/2 }}}
{{{ 4t = 10 }}}
Rob's time working alone is 2.5 hrs
Brother's time working alone is 10 hrs