Answer:
596
Explanation:
closest to 600.
this means we can narrow the first digit of all 5he three digits to be either 5 or 6:
5_ _
or
6_ _
however, when considering the closest number to 600 when starting with 6, we realise that we can only make 630.
when looking at starting with 5, we can make 590, much closer to 600.
so now, we have deduced it has to become 590
however, we can use 3 digits. so, using a 6, the next highest number, we can confidently rest our case.