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).

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