3 divs with exactly 33% width not fitting in parent

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This is a common problem but I can't figure out why it happens.I have a parent div and inside that div I have 3 divs with width set to 33% (exactly, not 33.3%!) and \[code\]display: inline-block\[/code\].In Chrome that works well, but in Mozilla and Opera it does not (I didn't test it in IE yet). I thought the problem might be in the algorithm browsers use to calculate pixel sizing from percentages. But when I checked the DOM metrics, I found that the parent's width is 864px and the child's is 285px (that's correct: 864 * .33 = 285.12). But why doesn't it fit in the parent? 285 * 3 = 855, that's 9px less than parent's width!Oh, yes, padding, margin and border for all divs set to 0 and DOM metrics confirm that.