An algebra problem by Denton Young

Algebra Level 1

A man puts a dollar under his bed on day 1, 2 dollars the next day, 3 dollars on day 3, and so on. After 15 days, he takes the stash and buys $100 worth of food. How much is left in the stash?


The answer is 20.

Denton Young by Denton Young , 47, USA

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2 solutions

after 15 days, he has n(n+1)/2 $

here, n=15(15 days)

so, (15 X 16)/2=120

he spends 100$ so, he has 20$ left

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Denton Young
Jun 1, 2015

(1+ 2+ 3...+ 15) = (1/2)(15)(15 + 1) = 120

120 - 100 = 20

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