Step acquisition

Hello In the following, I do not understand how the gain of 4 was arrived at. On 1 January 2008, A acquired a 50% interest in B for $60m. A already held a 20% interest which had been acquired for $20m but which was valued at $24m at 1 January 2008. The fair value of the NCI at 1 January 2008 was $40m, and the fair value of the identifiable net assets of B was $110m. The goodwill calculation would be as follows, using the full goodwill method: $m $m 1 January 2008 consideration 60 Fair value of interest held 24 84 NCI 40 124 Fair value of identifiable net assets (110) Goodwill 14 A gain of $4m would be recorded on the increase in the value of the previous holding in B.