Puzzle 125 - Pencils

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A bag contains some red pencils and some blue pencils. The ratio of red to blue pencils is 7:11. If the total number of pencils the bag is 90, what is the number of blue pencils?

(This question is borrowed from the book "Competitive Maths for middle school" by Vinod Krishnamoorthy) 

Answer : 55 pencils
 
If the total number of pencils is 7+11 = 18, then 11 is the number of blue pencils.
So the ratio of blue to total is 11:18
 
Since the total number of pencils is 90, number of blue pencils is
11/18*90 = 55 pencils

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