Question 739558
She took a total of {{{ n }}} tests
When her average was {{{ x }}}, she
had taken {{{ n - 1 }}} tests
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I cab say:
Total of her {{{ n-1 }}} test scores was
{{{ x*( n-1 ) }}}
The next test had a score of {{{ y }}}, 
so
the total is now {{{ x*( n-1 ) + y }}}
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The average score for her {{{ n }}} tests was:
{{{ ( x*( n-1 ) + y ) / n }}}