Question 1206870
B blue marbles at first
{{{5B/7}}} red marbles at first


x blue marbles removed and replaced with red marbles.
{{{B-x}}} blue marbles
{{{5B/7+x}}} red marbles


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 red marbles became 5/9 of all the marbles in the jar. 
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Two equations can be made.
{{{B+5B/7=(B-x)+(5B/7+x)}}}
{{{and}}}
{{{5B/7+x=(5/9)(B+5B/7)}}}



{{{12B/7=12B/7}}}
-
{{{45B/7+9x=5(12B/7)}}}
{{{45B+63x=35*12B}}}
{{{63x=35*12B}}}
7*9x=7*5*12B
{{{9x=60B}}}
{{{3x=20B}}}
Maybe something was missed.  Did you have choices to pick from?


x and  B must both be Natural numbers.  See from the {{{3x=20B}}}, B could be 3, and then x would be 20.