To add these fractions, we need to find a common denominator. The least common multiple (LCM) of 5, 25, and 20 is 100.

1/5 can be written as 20/100 (multiply numerator and denominator by 4).
4/25 can be written as 16/100 (multiply numerator and denominator by 4).
3/20 can be written as 15/100 (multiply numerator and denominator by 5).

Now, we can add the fractions:
20/100 + 16/100 + 15/100 = (20 + 16 + 15)/100 = 51/100

Therefore, 1/5 + 4/25 + 3/20 = 51/100.