so, "one third of remaing" is 21, this means that the "remaining" was three times as much: 21*3=63
This 63 is the 30 % of all the people, since they are the remaining of 100-70%.
so: 63=30/100x
we now multiply both sides by 100:
6300=30x
and divide by 30:
210
so all the people at the fair numbered 210!