Tips & Tricks Let the numbers be 9a and 9b, since 9 is a common factor, a and b are coprime. We know that 9a × 9b = H.C.F × L.C.M ⇒ 9a × 9b = 9 × 180 ⇒ ab = 20 The possible values of a and b are: (1,20), (4,5).The corresponding values of the two numbers would be (9,180), (36,45). Only 36 and 45 lie between 30 and 50, Therefore sum = 81.