gwen had slept 2 hours less than normal for each of the last 3 nights. If she's slept a total of 15 hours for all 3 nights, how many hours does she normally sleep per night?
Let X be the number of hours Gwen normally sleeps per night. If she slept two hours less than normal for each of the last three nights, then she slept X-2 hours for each of those nights.
And for all 3 nights, she slept a total of (X-2) + (X-2) + (X-2) = 15 hours.
So 3(X-2) = 15.
And 3X - 6 = 15.
And 3X = 15 + 6.
So 3X = 21.
Thus, the number of hours she normally sleeps nightly is X = 21 / 3 = <<7=7>>7 hours. Answer: \boxed{7}.