Answer:
Part (a)
The percentage ownership of Mr. John is 20% + (80% x 20%) = 36%
Part (b)
The percentage ownership of Mr. Brian is 30% + 30% = 60%
Part (c)
The percentage ownership of Mr. Charlie is 30% + 30% = 60%
Part (d)
The amount that could be recognized for the purposes of tax would be $0, as Mr. Brian owns more than half that is more than 50% of XYZ Corp. either directly or indirectly.