If all frogs, you should eat pests every day:

25*20=500 (piece)

Actually eat 620 a day, forget it:

620-500= 120 (piece)

This is because we count some toads that eat more as frogs, and each one eats less: 35-25= 10 (one).

So toad has: 120/ 10 = 12 (only)

Frog: 20- 12=8 (only)