Can you please answer, my parents are going to be so mad if I can't correct by like the afternoon right now its about 1:31.Anyone know inverse operations, cause I am confused, I looked online and it said Example: 20 + 9 = 29 can be reversed by 29 − 9 = 20 (back to where we started)
And the other way around:

Example: 15 − 3 = 12 can be reversed by 12 + 3 = 15 (back to where we started)
so like when do we do the 2nd way or can we just do either one of them???

An automobile assembly line operation has a scheduled mean completion time, μ, of 12 minutes. the standard deviation of completion times is 1.6 minutes. it is claimed that, under new management, the mean completion time has decreased. to test this claim, a random sample of 33 completion times under new management was taken. the sample had a mean of 11.2 minutes. assume that the population is normally distributed. can we support, at the 0.05 level of significance, the claim that the mean completion time has decreased under new management? assume that the standard deviation of completion times has not changed.