There are 18 points in a plane of which 5 are collinear. How many straight lines can be formed by joining them?

Explanation

To solve the problem of how many straight lines can be formed by joining 18 points in a plane, where 5 of these points are collinear, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Problem**:
We have a total of 18 points, and we need to find out how many straight lines can be formed by joining these points. To form a straight line, we need at least 2 points.

2. **Calculate Total Lines from All Points**:
The total number of ways to choose 2 points from 18 points is given by the combination formula \( C(n, r) \), where \( n \) is the total number of points and \( r \) is the number of points to choose.
\[ \text{Total lines} = C(18, 2) = \frac{18!}{2!(18-2)!} = \frac{18 \times 17}{2 \times 1} = 153 \]

3. **Calculate Lines from Collinear Points**:
Since 5 points are collinear, any line formed by choosing 2 points from these 5 points will not be counted as a unique line (as they all lie on the same line). Therefore, we need to subtract these combinations from our total.
\[ \text{Collinear lines} = C(5, 2) = \frac{5!}{2!(5-2)!} = \frac{5 \times 4}{2 \times 1} = 10 \]

4. **Final Calculation**:
Now, we subtract the number of lines formed by the collinear points from the total number of lines.
\[ \text{Valid lines} = \text{Total lines} - \text{Collinear lines} = 153 - 10 = 143 \]

5. **Conclusion**:
Therefore, the total number of straight lines that can be formed by joining the 18 points, considering that 5 of them are collinear, is:
\[ \text{Total straight lines} = 143 \]

### Final Answer:
The total number of straight lines that can be formed is **143**.