<PRE><B>Question 98457</B>
total weight of the rings = 18 * 5 = 90g 
total amount of silver = 10/100 *90 = 9g
Total amount of gold in all the 5 rings = 90 -9 =81g

Let the amount of silver added be x grams 
total weight of the mixture now = 90 + x

percentage of gold = 81/(90+x) *100

since the percentage of gold now is to be 75 

 81/(90+x) *100 = 75

81 * 100 = 75 (90+x)
27 * (3 * 25) * 4=  75(90+x)
canceling out 75 on both sided 

27* 4 = 90 +x
x = 108 -90
  =18g 

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