Central Tendency: Weighted Mean

At the Mountain School, Micchell is assigned a grade of $3.0$ or $4.0$ for each class he takes. His college potential is then defined as the average of his grades over all classes taken. After taking 40 classes, Micchell has a college potential of $3.975$. Unfortunately, he needs a college potential of at least $3.995$ to get into the South Harmon Institute of Technology. Otherwise, he becomes a rock. Assuming he receives a grade of $4.0$ in every class he takes from now on, how many more classes does he need to take in order to get into the South Harmon Institute of Technology?

Source: 2012-2013 Winter Online Math Open