Joshua teaches an SAT preparation class where each student takes a pre-test on the first day of class and a post- test on the last day of class. Here are summary statistics for the scores of a recent group of students:
Test
Mean
Standard deviation
Pre-test
1000
175
Pre
Post-test
1150
190
Let D represent the the difference between a randomly chosen student's pre- and post-test scores.
Find the standard deviation of D.