Question 905102
B and C together:
{{{ 1/12 + 1/t[c] = 1/4 }}}
Multiply both sides by {{{ 12t[c] }}}
{{{ t[c] + 12 = 3t[c] }}}
{{{ 2t[c] = 12 }}}
{{{ t[c] = 6 }}} 
It takes C 6 hrs to do the job alone