Amy had 30 CDs

The mean playing time of these was 42mins

She sold 5

The mean playing time of the 25 left is now 42.8

Calculate the mean playing time of the 5 CDs that Amy sold.

Method please.

2)She shutter speed S of a camera varies inversely as the square of the aperture setting, F. When F=8, S=125. Is this the same as the proportional formula where F=KS?