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Mrs Koh bought some pens, files and erasers. The ratio of the number of pens to the number of files to the number of erasers she bought was 1 : 2 : 3. The cost of each pen and eraser is $2.50 and $0.50 respectively. If she spent $60 on the pens and the erasers, how many files did she buy?

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  1. Say she bought x pens, 2x files, and 3x erasers. Adding up the cost of the pens and erasers, we have

    2.50x + 0.50(3x) = 60.00
    x = 15

    So, she bought 30 files

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