When you round to three decimals, two-thirds in decimal form is 0.667. In reality, 2/3 is written in decimal form as 0.666… The number six is followed by an ellipsis to show that it keeps coming back over and over again.
To turn a fraction into a decimal, you need to change the denominator to 10. In the example given above, three is the denominator. When you round it to three decimal places, you multiply it by 3.333 to get ten. To turn 2/3 into a decimal, you must first multiply 2 by 3,333. When you round to three decimals, three times 3.333 is equal to 10, and two times 3.333 is equal to 6.667. This makes the fraction 6.667/10. Dividing 6.667 by 10 yields 0.667.