Let a 95% confidence interval states that population mean is greater than 250 and less than 550. The sample mean and margin of error for the given data is:

400 & 150 respectively

$\text{95\% confidence interval }=(250,550).$

$\text{Sample mean }=\frac{250+550}{2}=400.$

$\text{Margin of error }=\frac{550-250}{2}=150.$

$\text{Sample mean }=400,\;\text{Margin of error }=150.$