if all three types have to be bought there is no solution because

if a= number of peacocks

b= number of pigeons

c=number of sparrows , these conditions for quantity and cost respectively must satisfy

a+b+c=100

15a+b+c/20=100

subtract 14a=19c/20 or a=19c/280

for a to be a whole number c has to be 280 which violates the fundamental condition of a,b,c each <100