<PRE><B>Question 2036</B>
I am not sure of my solution - but this is how i did it:
Assume that the rate at which the grass is eaten is partly dependent on the number of cows.
Then, 
nDays = k1 * nCows + k2
thus,
40 = 40k1 + k2
60 = 30k1 + k2
Solving for k1, k2;
k1 = -2, k2 = 120
Therefore,
in case of 20 cows:
nDays = 20*-2 + 120 = 80 days 
:) I hope I helped!
</PRE>