Answer:
The minimum of cows he needs are: 2
Step-by-step explanation:
There's a relation between each animal:
5 chickens equals 1 pig
3 pigs equals 2 sheep
5 sheep equals 2 cows
You can understand it as the following three abstractions:
5c = 1p (1)
3p = 2s (2)
5s = 2o (3)
Where:
c is for chickens
p is for pigs
s is for sheep
o is for cows
So now you have three equations with 4 variables. The next step is to obtain an equation that relates directly the variable c (chickens) with the variable o (cows). In order to do that from the equation 2 we obtain s in terms of p, as follow:
Then we replace s in the equation 3 and we obtain v in terms of p:
Now we replace v in the equation 1:
(4)
The equation 4 means that 1 chicken equals the fifteenth part of a cow. For this case the farmer needs 20 chikens, so we multiply per 20 each part of the equation 4:
As it is impossible to have 1.3333 cows, the answer is 2 cows approximately.