Anyone knows the answer to this question?
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My son has a big bag of marbles, each exactly 10mm in diameter. One day he made a triangular pyramid, stacking one marble one top of a triangle of three, three on top of a triangle of six, six on top of a triangle of ten, etc.
He ran out of marbles when the stack made a complete triangular pyramid.
I measured the stack with a ruler (accurate to 0.1 mm) and it was 900.0mm high.
How many marbles were in his stack?
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Wrong answers:
125,580
180,104
192,920
198,475
Last edited by a moderator: Apr 13, 2010
