Solution
If a number is divisible by each of 15, 21 and 28, then it should also be divisible by their LCM i.e. 420.
10000 when divided by 420 leaves remainder 340.
It means that the largest number which is divisible by 420 and is lower than 10000 is 10000 - 340 = 9660.
Thus, the largest four-digit number exactly divisible by 15,21 and 28 is 9660.
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