Question 1108740
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Let x = the number of the boys who were in 7th grade.


Let Z = the number of ALL students.


They want you determine the ratio x/Z  (it is almost the same as percentage).



    Let Y be the number of all 7th grade students.  


    Notice that {{{Y/Z}}} = {{{4/9}}}, as it follows from the condition.


    Also from the condition {{{x/Y}}} = {{{3/(3+5)}}} = {{{3/8}}}.



Now we have EVERYTHING to calculate 


    {{{x/Z}}} = {{{(x/Y)*(Y/Z)}}} = {{{(3/8)*(4/9)}}} = {{{12/72}}} = {{{1/6}}}.


Thus we just found the ratio  {{{x/Z}}} = {{{1/6}}}.


Hence, the percentage under the question is  {{{(1/6)*100}}} = {{{100/6}}} = 16.667 %  (approximately).
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 * * * Solved. * * *