Question 710628
5 ounces of perfume  consist of 95% alcohol .
 so , intital quantity of alcohol  = 0.95 * 5
                                  =4.75 onces
lets assume x ounces of some preservative has been added 
   now alcohol  in the preservative  = 75% of x
                                     =0.75 x
total quantity of alcohol   = 4.75 +0.75 x
 but total volume of perfume after adding preservative  =  5+x
  alcohol  in the total quantity  =  91 % of (5+x) 
                                   = 0.91*5+0.91 x
                                   =4.55 +0.91 x

 
         so   4.75 +0.75 x =4.55 +0.91 x
               4.75-4.55  =0.91 x -0.75 x
                    0.2  = 0.16 x
divide with 0.16 on both sides
0.2/0.16  = 0.16 x/0.16
1.25  = x
x= 1.25 onces
so amount of preservative added  = 1.25 ounces