I'm right at the same number as the rest of you, with the slightly different approach (not all that different, but one extra step to spell it out a bit more).
x=# people without hats
Two fifths of the people without hats are also without t-shirt, so:
200=(2/5)x
x=500 people without hats
That's one third of the people at the concert, so there are 1,500 people total