Question 906942
In words, you need to find:
( the number of folders that need to be filed ) / ( their rate of filing them together )
---------------
The 1st clerk's rate of filing:
( 12 folders ) / ( 1 min )
The 2nd clerk's rate of filing:
( 9 folders ) / ( 1 min )
--------------------------
Add their rates of filing to 
get their rate filing together:
Let {{{ t }}} = time in minutes 
to file 1 folder
{{{ 12/1 + 9/1 = 1/t }}}
------------------------
{{{ 21  = 1/t }}}
{{{ t = 1/21 }}}
-------------------
So, the rate working together is {{{ 1/t = 21/1 }}}
and, you need:
( the number of folders that need to be filed ) / ( their rate of filing them together )
{{{ 672 / (( 21/1 )) = 32 }}}
It will take them 32 minutes