A student scored the following marks: 78, 85, 69, and 90 in four subjects. What score is required in the fifth subject to achieve an average of 80 marks across all five subjects?​

Correct option is C

Given:
Marks in 4 subjects = 78, 85, 69, 90
Desired average for 5 subjects = 80 marks
Formula Used:
​$$\text{Average} = \frac{\text{Sum of all observations}}{\text{Number of observations}}$$​​
Solution:
Let the required 5th subject score be x
Sum of marks in first 4 subjects = 78 + 85 + 69 + 90 = 322
Then, total marks = 322 + x
​$$\frac{322 + x}5 = 80$$​​
322 + x = 400
x = 400 - 322 = 78