I try, had enough math in my studies...

If 10 birds are sold for one reyal, one duck is sold for 2 reyals and one chiken sells for 5 reyals

b= birds, d = ducks, c=chicken
h, j, k as the number you are looking for...

equations
h+j+k = 100 (100 animals)
h*0.1+j*2+k*5= 100 (100 ryial)

b=0.1
d=2
c=5

from the first equation, we can get: h = 100-j-k
put this in 2)
(100-j-k)*0.1+j*2+5*k=100
10-0.1j-0.1k+2j+5k=100
1.9j = 90 - 4.9k
j = 47.37 - 2.579k

Then, you have to solve it graphical...no way of doing it mathematical, with only 2 equations and 3 unknown numbers.