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The question is "What number is 40% of 10 y?".

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\"\"                                      (Write the proportion)

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\"\"                                  (Substitute: 10 y for b and 40 for p)

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\"\"                 (Multiply each side by 10y)

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\"\"                                (Cancel common terms)

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\"\"                                               (Simplify: \"\")\"\"

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40% of 10 y is 4 y.