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## 'Up and Down Staircases' printed from http://nrich.maths.org/

One block is needed to make an up-and-down staircase, with one
step up and one step down.

$4$ blocks make an up-and-down staircase with $2$ steps up and
$2$ steps down.

How many blocks would be needed to build an up-and-down
staircase with $5$ steps up and $5$ steps down?

Explain how you would work out the number of blocks needed to
build a staircase with any number of steps.