SOLUTION: A certain school has 720 pupils. The ratio of the number of senior pupils to the number of junior pupils is 4 : 5. How many junior pupils are there in the school?

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Question 923820: A certain school has 720 pupils. The ratio of the number of senior pupils to the number of junior pupils is 4 : 5. How many junior pupils are there in the school?
Answer by ewatrrr(24785) About Me  (Show Source):
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Hi

The ratio of the number of senior pupils to junior pupils is 4 : 5

Let x represent the constant of proportionality

4x + 5x = 720

9x = 720

x = 80

5x = 400, the number of junior pupils  in the school.