it's pretty clear
Pr[picking red marble] = 1/5
Pr[not picking red marble] = 4/5
odds of picking the red marble are the ratio of these or, using the common notation, 1:4 or 4:1 against.
This website has a quiz about probability and odds: Probability and Odds
It asks ten questions about the following scenario:
The first question it asks is:There are 20 marbles in a bag, 4 blue, 4 green, 4 red, 4 white and 4 black. Remember that all fraction answers should be given in the simplest form.
Which is pretty straightforward of course it's 4/20 or 1/5.What is the probability of pulling out a red marble?
Question 2 is about odds rather than probability and it asks:
For this, I worked out that the odds/ratio was 16:4 or (simplified) 4:1. However, the website lists the correct answer as 1/3 (or 1:3).What are the odds of pulling out a red marble?
Surely website is wrong here?