mathematics

hens = 2 legs
pigs = 4 legs

let hens be 'h' and pigs be 'p'

h + p = 50.....(1)
2h + 4p = 120....(2)

From equation (1)

h + p = 50
p = 50 - h ....(3)

Sub (3) into (2)
2h + 4(50-h) = 120
2h + 200 - 4h = 120
200 - 120 = 4h - 2h
80 = 2h
h = 40

Now we know that there are 40 hens. To find the number of pigs:
Sub 'h' into (1)

h + p = 50
40 + p = 50
p = 50 - 40
p = 10

ANSWER: there are 40 HENS and 10 PIGS