Explanation:
Odds against = failures / successes
Probability = successes / (successes + failures)
0.21 = cats / (cats + no cats)
1 / 0.21 = (cats + no cats) / cats
4.76 = 1 + (no cats / cats)
3.76 = no cats / cats
So the odds against are 3.76 to 1.
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