.
The Table below illustrates my attempts to get a solution by the "Trial and error" method.
Step by step I am approaching to the final solution.
It is presented in the last line.
n cows pigs chickens money
1 8 10 5 8*10 + 10*1.75 + 5*0.50 = 80 + 17.50 + 2.50 = 100
2 6 20 10 6*10 + 20*1.75 + 10*0.50 = 60 + 35.00 + 5.00 = 100
3 4 30 15 4*10 + 30*1.75 + 15*0.50 = 40 + 52.50 + 7.50 = 100
4 2 40 20 2*10 + 40*1.75 + 20*0.50 = 20 + 70.00 +10.00 = 100
5 0 50 25 0*10 + 50*1.75 + 25*0.50 = 0 + 87.50 +12.50 = 100
6 0 40 60 0*10 + 40*1.75 + 60*0.50 = 0 + 70.00 +30.00 = 100 (*)
The last line gives the final solution to your problem.