In a class of 28 students, 21 have a brother and 6 have a sister. There are 3 students who do not have any siblings. What is the probability that a student chosen randomly from the class has a sibling?

To solve this problem, let's break down the information provided and use it to calculate the probability.

### Step 1: Understand the Given Information

- Total number of students in the class: \(28\)
- Number of students who have a brother: \(21\)
- Number of students who have a sister: \(6\)
- Number of students who have no siblings: \(3\)

### Step 2: Determine the Number of Students with at Least One Sibling

The students who have no siblings are those without a brother or a sister. Thus, the number of students with at least one sibling is given by subtracting the number of students with no siblings from the total number of students:

\[
\text{Number of students with at least one sibling} = 28 - 3 = 25
\]

### Step 3: Calculate the Probability

The probability that a randomly chosen student has a sibling is the ratio of the number of students with at least one sibling to the total number of students:

\[
P(\text{has a sibling}) = \frac{\text{Number of students with at least one sibling}}{\text{Total number of students}} = \frac{25}{28}
\]

### Step 4: Simplify the Fraction (if needed)

The fraction \( \frac{25}{28} \) is already in its simplest form.

### Conclusion

The probability that a student chosen randomly from the class has a sibling is:

\[
\boxed{\frac{25}{28}}
\]

Would you like to know more details or have any other questions?

### Related Questions:

1. What is the probability that a randomly chosen student has a brother?
2. What is the probability that a randomly chosen student has a sister?
3. How many students have both a brother and a sister?
4. If another student is added to the class who has a brother, how would the probability change?
5. What is the probability that a randomly chosen student has neither a brother nor a sister?

### Tip:

When calculating probabilities, always ensure that the events considered are mutually exclusive and collectively exhaustive to avoid any overlap or omission.

Calculate the probability that a randomly chosen student in a class of 28 has a sibling. This problem involves basic probability concepts and simple arithmetic, making it suitable for students in Grades 5-7.

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