A teacher has 21 red pens. If the ratio of red pens to blue pens she owned was 3:1 , how many pens did she have total?
1 answer:
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We can employ a simple repeated decimal trick:
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Alternatively, we can compute the partial sum of the series.
As , the second term vanishes and we're left with
. Notice that this is really just a more formal version of the earlier trick.