Question 1041636
It's a little bit strange for a word problem. 
You can say for sure he has a GPA of 3.2
He wants to have a GPA of 3.3
He took 20 credits to get the 3.2 GPA
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The strange part is that the GPA of 3.5
must only apply to the credits he gets 
from now on, even though that number
doesn't mean very much to him
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Let {{{ n }}} = the number of credits he needs
from this point on.
Let {{{ s[2] }}} = the sum of his new grades
I can say:
[ GPA( new credits only ) ] = [ sum of the new grades ] / [ number of credits ]
{{{ 3.5 = s[2] / n }}}
{{{ s[2] = 3.5n }}}
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Let {{{ s[1] }}} = the sum of his grades at this point
So, at this point, I can say:
{{{ 3.2 = s[1] / 20 }}}
{{{ s[1] = 64 }}}
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Now I can say:
{{{ 3.3 = ( s[1] + s[2] ) / ( 20 + n ) }}}
{{{ 3.3 = ( 64 + 3.5n ) / ( 20 + n ) }}}
{{{ 3.3*( 20 + n ) = 64 + 3.5n }}}
{{{ 66 + 3.3n = 64 + 3.5n }}}
{{{ .2n = 2 }}}
{{{ n = 10 }}}
He needs 10 credits that have a GPA of 3.5
in order to have a 3.3 GPA
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check:
{{{ s[2] = 3.5n }}}
{{{ s[2] = 3.5*10 }}}
{{{ s[2] = 35 }}}
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{{{ s[1] = 64 }}}
{{{ s[1] + s[2] = 35 + 64 }}}
{{{ s[1]  + s[2] = 99 }}}
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{{{ 3.3 = 99 / ( 20 + n ) }}}
{{{ 3.3 = 99 / ( 20 + 10 ) }}}
{{{ 3.3 = 99 / 30 }}}
{{{ 99 = 99 }}}
OK