Question 914259
Average grade = ( total of the grades ) / ( total number of tests )
Total number of tests = {{{ 4 }}}
Let {{{ x }}} = his grade on the 4th test
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His ( total of grades ) for the first 3 tests was:
( average grade ) x ( number of tests ) = total of grades
{{{ 85*3 = 255 }}}
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I can say:
{{{ 88 = (( 85*3 + x )) / 4 }}}
{{{ 88 = (( 255 + x )) / 4 }}}
{{{ 255 + x = 4*88 }}}
{{{ x = 352  - 255 }}}
{{{ x = 97 }}}
He needs a 97 on the 4th test
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check:
{{{ 88 = (( 85*3 + 97 )) / 4 }}}
{{{ 88 = (( 255 + 97 ))/4 }}}
{{{ 88 = 352 / 4 }}}
{{{ 88 = 88 }}}
OK