Marks of a student in four subjects are 90 ,85 ,80 and 75 .How much he should score in the fifth subject to have an average of 80 ?

Answer

To find out how much the student should score in the fifth subject to achieve an average of 80, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Marks in the First Four Subjects**:
The marks in the four subjects are 90, 85, 80, and 75.

2. **Calculate the Total Marks Required for an Average of 80**:
The formula for average is:
\[ \text{Average} = \frac{\text{Total Marks}}{\text{Number of Subjects}} \]
To find the total marks needed for an average of 80 over 5 subjects:
\[ \text{Total Marks} = \text{Average} \times \text{Number of Subjects} = 80 \times 5 = 400 \]

3. **Calculate the Sum of the Marks in the First Four Subjects**:
Add the marks together:
\[ 90 + 85 + 80 + 75 = 330 \]

4. **Set Up an Equation to Find the Marks in the Fifth Subject**:
Let \( x \) be the marks in the fifth subject. The equation will be:
\[ 330 + x = 400 \]

5. **Solve for \( x \)**:
Subtract 330 from both sides to find \( x \):
\[ x = 400 - 330 = 70 \]

6. **Conclusion**:
The student must score **70 marks** in the fifth subject to achieve an average of 80.