Let X be the greatest number of 4
digits, which when divided by 15,20,28 leaves in each case the remainder 2. What will be the sum of digits of X
LCM(15,20,28) = 420
10000/420 = 23.8
So 420*23 = 9660 is the largest 4-digit number that is exactly divisible by 15,20,28.
so finish it off.