A student obtained 75%, 80% and 85% in three subjects, if the marks of another subject are added then the number cannot be less than ______?

70%

80%

60%

None of these

Let's assume the maximum marks in each subject are 100. Then, the student's total marks in the three subjects are:

75 + 80 + 85 = 240

If the marks of another subject are added, the total marks become:

240 + x (where x is the marks obtained in the fourth subject)

The average percentage cannot be less than 60% because:

If the student scores 0 in the fourth subject, the total marks would be 240, and the average percentage would be (240/400) x 100% = 60%.
If the student scores any marks in the fourth subject, the average percentage would be higher than 60%.