In sampling without replacement from a population of 900, it’s found that the standard error of the mean, σx, is only two-thirds as large as it would have been if the population were infinite in size. That is the approximate sample size?
For a small population:
where is the population standard deviation, N is the population size and n is the sample size.
is the finite population correction factor.
For a large population:
The given value of the finite population correction factor is and you're also given N = 900:
.