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A box has some red and black racket balls. The Ratio of number of red balls to the number of black balls is 4:5. If the bag contains 100 balls, then how many blue balls are there in the bag?

This kind of situation can be solved using cross multiplication that is;

4/5=y/100

By cross multiplying we get;

5y=400

solving for y we get;

y=80

Thus, there are 80 red balls in the sack

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