[1,000/(800]1/20 - 1

The brackets are not balanced   

anyway you can just type it straight into the calc on the home page.

Only use round brackets

[1,000/(800]1/20 - 1

becomes

(1000/800)*1/20 - 1

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actually now I am looking at it it looks simple

(1000/800)*1/20 - 1

=5/4)*1/20 - 1

1/4*1/4-1

1/16-1

-15/16