Get the prime factorizations of all three numbers HCF = 145 = 5*29 LCM = 2175 = 3*5*5*29 one number = 725 = 5*5*29 The other number must contain a factor of 3 for 2175 to be the LCM. The other number must contain factors 5 and 29 for 145 to be the HCF. So the other number is 3*5*29 = 435 Edwin