Question 716023
Add their rates of working:
An experienced surveyor's rate = ( 1 road bed ) / ( 3 hrs )
An apprentice surveyor's rate = ( 1 road bed ) / ( 7 hrs )
Let {{{ t }}} = their time in hrs with both working
{{{ 1/3 + 1/7 = 1/t }}}
Multiply both sides by {{{ 21t }}}
{{{ 7t + 3t = 21 }}}
{{{ 10t = 21 }}}
{{{ t = 2.1 }}}
Working together, it will take them 
2.1 hrs to complete the job.