Let
x=duck
y=chicken
z=bird

Given:
1. selling prices:
x=2
y=5
z=.1

2. Quantity:
x=?
y=?
z=100

Therefore the equation to get a total of 100 selling price is:
2x+5y+.1(100)=100

Simplify:
2x+5y+10=100
2x+5y=90

Then you can subsitute any number to the variable to get the combinations (mix) and have a total 100 riyals.