If you know that you don't know, then guess

Guess - to form an opinion based on insufficient information; to arrive at a conclusion by conjecture, chance or intuition.

Guessing is often discouraged and downplayed as a means to determining a solution to a problem.

Personally, I think it's a legitimate option. Looking at test-taking strategy courses and websites, you will find plenty of math evidence to show that the odds are better if you guess than if you leave an answer blank.

In most cases, you get 0 points for leaving an answer blank, and you will definitely get more than 0 if you guess the right answer.

If there are 5 options, then you have a 1:5 chance (20%) of getting the answer if you guess. And you can increase your odds if you can eliminate several of the 5 options as impossible or unlikely.

Some tests penalize you for guessing and getting the wrong answer, but the penalties are unlikely to equal the value of getting the occasional answer correct from guessing. To determine the exact value, we'd need to know more information about the type of test and the penalty strategy. For example, guessing on an essay question is much more complex ...

But let's forget the "tests" and talk about real life instead. Do you guess very often?

I guess you do. For example, we guess about what to order in a restaurant - choosing something that sounds good. If you've had it before by the same chef at the same place, you might be making a an educated guess, but it's still a bit of a gamble compared to walking down a buffet line or sticking your finger into the cooking pot at home ... hence the plaintive cries of  "I don't know what to get!"

Let's take another term called guesstimate. Maybe using this word helps a bit. It certainly sounds better, with a dollop of estimate making the guess seem more credible. It allows us to use our previous experience, a bit of evidence and some reasoning to construct an educated guess.

I guess if you want to learn more about guesstimating, you could buy a book on the subject from Amazon. My guess is that it would cost you about $10-15 dollars and be delivered in 2-5 days, right to your home, or in a minute or two to your Kindle.

I found another book whose title can be roughly summarized as  "How Many Licks on an Ice Cream Cone, or how to estimate d*** near anything". That's guessing - number of licks on a cone...

After some consideration, I came up with an example where guessing causes nothing but grief ...