In an examination there are three subjects of 100 marks each. A student scores 60% in the first subject and 80% in the second subject. He scored 70% in aggregate. His percentage of marks in the third subject is

Options

A. 80
B. 60
C. 65
D. 70

Answer

To find the percentage of marks the student scored in the third subject, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the total marks and percentages**:
- There are three subjects, each out of 100 marks.
- The total marks for all subjects combined is \( 100 + 100 + 100 = 300 \).

2. **Calculate the marks scored in the first and second subjects**:
- For the first subject, the student scored 60% of 100 marks:
\[ \text{Marks in first subject} = 60\% \times 100 = \frac{60}{100} \times 100 = 60 \text{ marks} \]
- For the second subject, the student scored 80% of 100 marks:
\[ \text{Marks in second subject} = 80\% \times 100 = \frac{80}{100} \times 100 = 80 \text{ marks} \]

3. **Calculate the total marks scored so far**:
- The total marks scored in the first and second subjects is:
\[ \text{Total marks in first and second} = 60 + 80 = 140 \text{ marks} \]

4. **Calculate the aggregate marks required for 70%**:
- The student scored 70% in aggregate across all three subjects:
\[ \text{Aggregate marks} = 70\% \times 300 = \frac{70}{100} \times 300 = 210 \text{ marks} \]

5. **Set up the equation to find the marks in the third subject**:
- Let \( x \) be the marks scored in the third subject. The equation will be:
\[ 140 + x = 210 \]

6. **Solve for \( x \)**:
- Rearranging the equation gives:
\[ x = 210 - 140 = 70 \text{ marks} \]

7. **Calculate the percentage of marks in the third subject**:
- To find the percentage in the third subject:
\[ \text{Percentage in third subject} = \left( \frac{x}{100} \right) \times 100 = \left( \frac{70}{100} \right) \times 100 = 70\% \]

### Final Answer:
The percentage of marks in the third subject is **70%**.