Question 888384
Eric's rate of defeating soldiers is:
( 3 soldiers / 10 min ) = ( 3/10 soldiers / 1 min )
After 1 minute, there are {{{ 30 - 3/10 = 297/10 }}} soldiers left
Add their rate of defeating soldiers and
let {{{ t }}} = time in hours to defeat remaining soldiers 
{{{ 3/10 + 5/6 = ( 297/10 ) / t }}} 
{{{ 3/10 + 5/6 = 297/ ( 10t ) }}} 
Multiply both sides by {{{ 30t }}}
{{{ 9t + 25t = 3*297 }}}
{{{ 34t = 891 }}}
{{{ t = 26.206 }}}
{{{ .206*60 = 12.36 }}}
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Fighting together, it takes them 26 min 12 sec to
finish defeating soldiers
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check:
{{{ 3/10 + 5/6 = 297/ ( 10t ) }}} 
{{{ 3/10 + 5/6 = 297/ ( 10*26.206) }}} 
{{{ .3 + .8333 = 296 / 262.06 }}}
{{{ 1.1333 = 1.1295 }}}
close enough