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