Question 908426
{{{ mn + 5n^2  - 4m - 20n }}}
change the grouping:
{{{ ( 5n^2 - 20n ) + ( mn - 4m ) }}}
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Factor {{{ 5n }}} out of the 1st group
{{{ 5n*( n - 4 ) + ( mn - 4m ) }}}
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Factor {{{ m }}} out of the 2nd group:
{{{ 5n*( n - 4 ) + m*( n - 4 ) }}}
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Now {{{ n - 4 }}} can be factored out
the 1st and 2nd terms
{{{ ( n - 4 )*( 5n + m ) }}}
This is  as far as I can go with it