As shown in the figure (click to enlarge):
BTW: When I proved for 1 time, I added a non-negative condition of f(x) because I couldn't prove it.
I won't delete it, keep it and write a lot for your reference:
See the figure below (click to enlarge):
If all frogs, you should eat pests every day:
25*20=500 (piece)
Actually eat 620 a day, forget it:
620-500= 120 (piece)