Amy had 30 CDs
The mean playing time of these 30 CDs was 42 minutes.
Amy sold 5 of her CD.
The mean playing time of the 25 CDs left was 42.8 minutes.
Calculate the mean playing time of 5 CDs that Amy sold.
+33615 Total time of all 30 can be written two ways:
25*42.8 + 5*m where m is mean time of the 5 CDs that were sold, and
30*42
So 25*42.8 + 5m = 30*42
See if you can find m from this.
.
+118609 Total playing time for all 30 is 30*42 = 1260 minutes
Total playing time for 25 left is $${\mathtt{25}}{\mathtt{\,\times\,}}{\mathtt{42.8}} = {\mathtt{1\,070}}$$
Total playing time for the 5 gone is $${\mathtt{1\,260}}{\mathtt{\,-\,}}{\mathtt{1\,070}} = {\mathtt{190}}$$
Average playing time for the 5 is $${\frac{{\mathtt{190}}}{{\mathtt{5}}}} = {\mathtt{38}}$$ minutes
