To take the LCD of two farctions, one must take the LCM of its denominators.
The prime factors of 5 is 1 and 5; Of s, is s and 1
Thus, their LCM = 1 x 5 x s = 5s
Thus the LCD of 4/5 and 4/s is 5s.
The new fractions therefore are:
4s/5s and 20/5s.
So 4s/5s + 20/5s = (4s + 20) / 5s