Multiply 2/3 and your number. If you have a whole number, convert it to a fraction by putting it over a denominator of 1. When multiplying fractions, calculate numerator times numerator, then denominator times denominator. Reduce the resulting fraction with some quick division. For example, to find two-thirds of 18, write 2/3 x 18/1. Multiply the numerators and then the denominators to get the fraction 36/3. Divide the numerator by the denominator: 36 / 3 = 12.
When working with a mixed number, or a whole number and a fraction, first change it to an improper fraction: Multiply the denominator and the whole number. Add that to the numerator. Write the sum over the original denominator. For example, to convert 2 5/6: 6 x 2 = 12; 12 + 5 = 17. The improper fraction is 17/6.
Change two-thirds to a decimal and then multiply the decimal and your number. To convert 2/3 to decimal, divide the numerator by the denominator: 2 / 3 = 0.66666 ... 7, which you can round to 0.67. For example, to find 2/3 of 21: 0.67 * 21 = 14.07. Round to the nearest whole number: 14.