Question 669852
Let {{{ c }}} = the number of correct solutions
Let {{{ i }}} = the number of incorrect solutions
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{{{ c + i = 82 }}}
{{{ i = 82 - c }}}
{{{ 5c - 2i  = 25 }}}
{{{ 5c -2*( 82 - c ) = 25 }}}
{{{ 5c - 164 + 2c = 25 }}}
{{{ 7c = 189 }}}
{{{ c = 27 }}}
and
{{{ i = 82 - c }}}
{{{ i = 82 - 27 }}}
{{{ i = 55 }}}
He solved 27 correctly
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Your mistake is the 6th line from the end
{{{ 10c }}} should be {{{ 2c }}}