3/5 of the students ride the bus, what percent does not?

The answer is:  " 40 % " .
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Note:
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  If  "3/5" of the students ride the bus, then "2/5" do not ride the bus".

Since:  "3/5" + "2/5" = "(3 + 2)" / 5 = 5/5 = "1" (as in "1 whole"). 

1 – (3/5) = (5/5) – (3/5) = (5 – 3) / 5 = 2/5. 
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Now, express:  "2/5" as a percentage ; 

       →  "2/5"  = "x / 100 " ; Solve for "x" 

Cross-multiply:  Note:

→  Given:  ""  =  "" ; 

                  →  " bc = ad" ;  {" b0 ; d0 "} .
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As such:  

→   ""  =  "" ; 

                  →  " 5x = (2)*(100) " ;  →  Solve for "x" ; 

                  →  " 5x = 200 " ;  

                  →  Divide each side of the equation by "5" ;
               to isolate "x" on one side of the equation; & to solve for "x" ;

                  →  " 5x / 5  = 200 / 5 " ; 
         
             to get:   
 
                  →    "  x = 40 " ; 
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So; we have:

       →  "2/5"  = "40 / 100 " ;
 
       →  "2/5"  =  " 40 % " .
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The answer is:  " 40 % " .
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