Maths puzzling question - Mean?

Question

Amy had 30 CDSs .
the mean playing time of these 30CDs was 42 minutes
Amy sold 5 of her CDs.
The mean playing time of the 25CDs left was 42.8 minutes.
Calculate the mean time of the 5 CDs that Amy sold

Answer 1

the mean playing time of these 30CDs was 42 minutes

total playing time = 30 * 42

= 1260
Amy sold 5 of her CDs.
The mean playing time of the 25CDs left was 42.8 minutes.

= 1070

the difference betwwen 1260 and 1070 is the total playing time for the 5 cd = 190

the mean playing time for the 5cd is 190/ 5 = 38 min

Answer 2

Let S30 be the sum of the playing times and M30 be the mean.

Then S30 = 30*M30 total playing time for the 30 CDs

similarly:

S25 = 25*M25, playing time for 25 CDs

S5 = 5*M5, playing time for 5 CDs

S30 = S25+S5,

you know, or can get, S30, S25 and so you can get S5 from that last equation.
Then M5=S5/5.

Answer 3

With each group of CDs you can use the mean and number of them to find their total playing time. Take it from there, and try to do it yourself before looking at anyone's detailed answer that they leave here.