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I am confused as to how I was meant to know E for a X^2 test in the following question:

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2.i and 2.ii give no more information.

Not knowing what to do I used E as 0 for the first row and 40 (3+37) for the second row, but that was just a guess. It was wrong and the correct answer for E is 20.

I suspect this has something to do with the 20 fields but I can't beleive that I'm just meant to know they expect one butterfly per field...

How was I meant to know E?

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    $\begingroup$ I'm voting to close this question as off-topic because it would be better suited for the statistics SE site, Cross Validated $\endgroup$
    – theforestecologist ♦
    Commented Feb 22, 2017 at 15:29

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The expected value is what the values would be if the tested conditions had no effect on the sample distribution. In your case, you are testing the effect of herbicides on butterfly populations. If the presence of herbicides wouldn't matter you would expect an equal number of butterflies on fields with herbicides as without.

So, the total number of butterflies being $37 + 3 = 40$, you would expect $\frac{40}{2} = 20$ butterflies on fields without and $\frac{40}{2} = 20$ on fields with herbicides.

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