We hypothesise that all 36 animal are dogs so the number of legs are:
$36 x 4=144$(legs)
So the number of legs redundant are:
$144-100=44$ (legs)
The reason why it's redundant because every 2 legs of chicken now exchange by 4 legs of dogs which is redundant: $4-2 =2$ legs
So the number of chickens are:
$44:2=22$ chickens
The number of dogs are:
$36-22=14$ dogs