7 - 10 year olds

Mary has some train wagons filled with cubes, like this one, with 6 cubes in it.

She wants to build a train.

Her brother Keenan wants some cubes to build his own train wagon with 6 cubes.

They come to the understanding that each 5 wagons that go by, Keenan will grab a cube from the last wagon that Mary has added; this wagon will have a cube less than the other wagons in the train.

When Keenan grabs the last cube to complete his vagon,

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a) How many vagons has Mary’s train got?

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b) How many complete vagons has the train got?

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c)How many cubes are there in total, with all the wagons, on the train?