Explanation:

- The average marks of 20 students are initially calculated as 70.

- Total initial marks = 20 students × 70 marks = 1400 marks.

- Two students' marks were noted incorrectly as 54 and 74.

- The correct marks for these students are 64 and 84.

- Calculate the difference for each student:

- First student: 64 - 54 = 10 marks

- Second student: 84 - 74 = 10 marks

- The total correction to be added: 10 + 10 = 20 marks.

- Revised total marks: 1400 + 20 = 1420 marks.

- Correct average = Total revised marks ÷ number of students = 1420 ÷ 20 = 71.

- Correct Answer: Option 3: 71