One letter is chosen at random from the word isosceles what is the probability of choosing?

It's not specified what exactly you want to find the probability of, but as a general rule:

The Word "isosceles" has nine letters, so your denominator will be nine because that is the number of possible choices.

To find the numerator, determine how many letters meet the criteria or criterion you want to satisfy. For example:

If you want to find the probability of choosing the letter s: there are three "s"s in the Word "isosceles," so your numerator would be three, giving you a fraction of 3/9, which reduces to 1/3.

If you want to find the probability of choosing a vowel: there are four vowels (i, o, e, e), so your probability would be 4/9.

The same principle applies regardless of how the question ends (the probability of choosing ____). Remember that if you want your answer as a percent, you can simply divide the fraction and multiply by 100%.