Question 674345
Add their rates of working
Anthony's rate is ( 1 wall ) / ( 10 hrs )
Helper's rate is ( 1 wall ) / ( 15 hrs ) 
Let {{{ t }}} = time in hours for them to do it together
{{{ 1/10 + 1/15 = 1/t }}}
Multiply both sides by {{{ 30t }}}
{{{ 3t + 2t = 30 }}}
{{{ 5t = 30 }}}
{{{ t = 6 }}}
It takes them 6 hrs working together