The greatest 4 digits number is 9999
The LCM of 12, 18, 21, 28 is 252
On dividing 9999 by 252 the remainder comes out to 171
Required number = 9999 - 171 = 9828
Let the numbers are 3x and 4x
So HCF = x
∴ HCF × LCM = Product of numbers
⇒ x × 120 = 3x × 4x
⇒ x × 120 = 12x^{2}
⇒ 120 = 12x
x = 10
∴ Numbers are 30 and 40
∴ Sum of two numbers = 30 + 40 = 70