Two numbers are in ratio 4 : 5 and their LCM is 180. The smaller number is

Answer: Option C

Let two numbers be 4x and 5x
Their LCM = 180 and HCF = x
Now,
1^st number × 2^nd number = LCM × HCF
Or, 4x × 5x = 180 × x
Or, 20x = 180
Or, x = 9
Then, the smaller number = 4 × 9 = 36