Let's assume the total number of tomatoes in the bag is represented by 'x'. 

We are given that 399 tomatoes were rotten, which is 15% of the total number of tomatoes. 

We can set up the equation:
15% of x = 399

To solve for 'x', we can convert 15% to its decimal form by dividing it by 100:
0.15x = 399

To isolate 'x', we divide both sides of the equation by 0.15:
x = 399 / 0.15

Evaluating the expression:
x = 2660

Therefore, there were 2660 tomatoes in the bag.