If Miranda wants to buy 3 boxes of glue for $10 each and she pays $20 for each, how much change will she get back?
1 answer:
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14 + 28 + 42 + 56 + … 280
= 14 (1 + 2 + 3 + 4 + … + 20)
= 14 × 20 × 21 / 2
= 2940
where we use the well-known identity,