Set X for Daniel to eat hay every day; Calves eat haystacks. So there is.

12x+ 12y= 1

12x- 12y= 1/2

D: x=3/48

y= 1/48

That is to say: Daniel eats 3/48 of this pile of grass every day, and calf eats 1/48 every day.

If Daniel eats alone, it is exactly 1/(3/48)= 16 (days).

If the calf is eaten alone, it is exactly 1/( 1/48)=48 (days).