Question 1055498
The rate of charging for the wind 
generator is [ 1 pack ] / [ 24 hrs ]
( I can call the 20 storage batteries 1 pack
because the problem is only about the 20
batteries, never more & never less. )
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The fraction of charging after 6 hrs is:
{{{ 6*( 1/24 ) = 1/4 }}}
There is {{{ 1 - 1/4 = 3/4 }}} of the charging job 
left to do
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Now you have 2 generators charging the pack.
Add their rates of charging.
Let {{{ t }}} = time to do the {{{ 3/4 }}} of the charging job
{{{ 1/24 + 1/36 = ((3/4))/t }}}
Multiply both sides by {{{ 72t }}}
{{{ 3t + 2t = 3*18 }}}
{{{ 5t = 54 }}}
{{{ t = 10.8 }}}
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10.8 additional hours are needed 
to charge the 20 batteries
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check:
{{{ 1/24 + 1/36 = ((3/4))/t }}}
{{{ 1/24 + 1/36 = ((3/4))/10.8 }}}
{{{ .0416667 + .027777 = .069444 }}}
{{{ .0694437 = .069444 }}}
error due to rounding off, I think
OK