In a firm there are cows and hens. If heads are counted there are 180, If legs are counted there are 420.The total numbers of cows in the firm is.. A.130 B. 50 C. 150 D. 30

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Let

h = number of hens and 

c = number of cows. We have:

h+c = 180 (heads)

2h+4c = 420(legs)

Quick trick:

= If all 180 animals were hens, legs = 180×2=360

= Actual legs = 420, so there are 420−360 = 60 “extra” legs.

= Each cow has 2 more legs than a hen, so number of cows = 60÷2 = 30

Answer: 30 (Option D)