Question 961797
[ average ] = { sum of test scores ] / [ number of tests ]
Let {{{ x }}} = her score on the 4th test
{{{ 80 = ( 63 + 77 + 89 + x ) / 4 }}}
{{{ 80 = ( 229 + x ) / 4 }}}
{{{ x + 229 = 320 }}}
{{{ x = 91 }}}
She has to get a 91 on the 4th test