There are chicken and rabbits on a farm. you count 98 legs and your friend counts 30 heads. how many chickens and how many rabbits are there?

Chickens have 1 head and 2 legs
rabbits had 1 head and 4 legs

total heads is 30
c+r=30
total legs is 98
2c+4r=98
divide by 2
c+2r=49

multiply top equation by -1 and add to 2nd equation

-c-r=-30
c+2r=49 +
0c+1r=19

r=19

sub back
c+r=30
c+19=30
minus 19
c=11

11 chickens
19 rabbits