let two numbers be 4x and 5x
Their LCM = 180 and HCF = x
$$1st number\times 2nd number = LCM\times HCF$$
$$4x\times 5x= 180\times x$$Â
$$420x = 180$$
 x = 9
 1st number = $$4\times 9$$ = 36
2nd number =$$5\times 9$$ = 45
    sum = 81
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