Since she paid $300 postage, the hats cost $2685-$300 or $2385.
If she could have bought them all at $6 each that would be the most
number of hats.  Then she would have 2385/6 = 397.5 hats.  'Course 
she can't buy half a hat and also she can only get $6 hats in 
batches of 10, so she bought 390 hats, 39 batches of 10 each at $60 
per batch for $2340 and she spent the other $45 on single hats at 
$7.50 each and $45/$7.50 = 6 hats

So the most number of hats she could have bought would have been 
396 hats.

Edwin

With $300 for postage, cost of hats = $2,685 – 300, or $2,385

Obviously, more than 10 were bought, so  = 39 groups of 10, with a remainder of $45. 

With each hat costing $7.50, we get: , or 6 additional hats

Therefore, 39 groups of 10, or 390, plus 6, for a total of 390 + 6, or  hats were bought 

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