This is a number you will have memorized in grade school. If not, you can always perform long division of 2 by 3 to find it---or ask your calculator.
2/3 = 0.6666_6 . . . . . . repeating 6s indefinitely
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Most folks get tired of writing 6s after a couple million of them, so they round it off to 0.67 or 0.667. These rounded values are NOT exactly 2/3.