2/3 of 800
The word 'of' generally means multiply, so then
(2/3) * 800
800 is like 800/1, the fraction, and when multiplying fractions we multiply the top of the first and the second together, and multiply the bottom of the first and second together (numerator, denominator).
[tex] \frac{2}{3} * \frac{800}{1}
=\frac{1600}{3}
=533.333333333333[/tex]
You don't really need the 800/1, but that might make it more clear exactly what's happening.
Edit: So I'm not sure if it's just me or that LaTeX formatting actually doesn't render. If not,
(2\3)*(800/1) = 1600/3 = 533.333333
