In a class containing k students, the teacher calculated the students’ mean grade on a test to be 50. Unfortunately, the teacher made a mistake in the calculation and recorded one student’s score of 20 as 70. If the mean after the error was fixed is one more than the number of students in the class, what is k?

The problem can be found in Data Analysis category. Earlier when I solved the question I formed a wrong equation by interchanging 20 and 70, I got 50 as answer which was correct as per website. But when I was doing it again, I formed the correct equation (as per my judgement) it became a very complicated equation and the solutions were not whole numbers. Can anyone shed some light on this question?