Statistics — sample size for a test of difference of means

The mean height of $50$ male students who showed above average participation in college athletics was $68.2$ inches with a standard deviation of $2.5$ inches, while $50$ male students who showed no interest in such participation had a mean height of $67.5$ inches with a standard deviation of $2.8$ inches. In order that the observed difference in the mean height to be significant at $5\%$ level of significance, the sample size of each of the two groups has to be increased by (the critical value of $Z$ at $5\%$ level of significance is $1.645$)