The margin of error is given by: ME = critical value*standard error. Standard error is `sigma/sqrt n` .
Here, sigma is known to be 68. The number of samples is 50. This gives standard error as `68/sqrt 50 = 9.6166` . For the 95% confidence interval the critical value is 1.96. The margin of error is 1.96*9.6166 = 18.84
The required margin of error is 18.84