One day, a person went to a horse racing area. instead of counting the number of humans and horses, he counted 72 heads and 192 legs. how many humans and horses were there?

H-human a-horse
h+a= 72
2h+4a=192

then do h=72-a
substitute that into the second equation
2(72-a)+4a=192

multiple out
144-2a+4a=192
2a=48
a=24
substitute a into the first equation
h+24=72
h=48
humans- 48
horses- 24