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What would you need to multiply by to get the same answer as calculating $10\%$ and then adding it on?
Try some different values for the sum invested. It might help to use a spreadsheet.

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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