There are 10 points in a plane and 4 of them are collinear. Find the number of straight lines joining any two of them.

Answer

To solve the problem of finding the number of straight lines that can be formed by 10 points in a plane where 4 of them are collinear, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Total Points and Collinear Points**:
We have a total of 10 points, and among these, 4 points are collinear.

2. **Calculate the Total Lines from All Points**:
To find the total number of lines that can be formed by selecting any 2 points from the 10 points, we use the combination formula \( nCk \), where \( n \) is the total number of points and \( k \) is the number of points to choose.
\[ \text{Total lines} = \binom{10}{2} = \frac{10 \times 9}{2 \times 1} = 45 \]

3. **Calculate the Lines from Collinear Points**:
The 4 collinear points can form lines, but since they are collinear, they only form 1 unique line. However, if we calculate the number of lines that can be formed by choosing any 2 from these 4 points, we get:
\[ \text{Collinear lines} = \binom{4}{2} = \frac{4 \times 3}{2 \times 1} = 6 \]
But since all these 6 combinations represent the same line, we only count it as 1 line.

4. **Adjust the Total Lines Calculation**:
To find the actual number of unique lines, we subtract the 6 lines formed by the collinear points from the total lines and then add back the 1 unique line formed by those collinear points:
\[ \text{Unique lines} = 45 - 6 + 1 = 40 \]

5. **Final Answer**:
Thus, the total number of unique straight lines that can be formed by joining any two of the 10 points is:
\[ \text{Total unique lines} = 40 \]

### Final Result:
The number of straight lines joining any two of the 10 points is **40**.