Because you are asked of odds against smth. Then odds are P of not happening : P happening. If you were asked of odds of picking up a spade then your answer would be correct.

If you think about sports betting – the Belmont Stakes are being run a week from Saturday, not that anyone on _ this _ forum will care – they always quote the odds against a particular horse, driver, team, whatever winning, and they’re generally greater than 1:1. So the odds against getting a spade are 39:13 (39 non-spades versus 13 spades) or 3:1; the odds in favor of getting a spade are 13:39 or 1:3.