Valuation of goodwill in admitting new partner

A and B are partners with Capitals of 10,000 and 20,000 respectively and sharing profits equally. They admitted C as their third partner with one-fourth's of profits of the firm on the payment of 12,000. The amount of hidden goodwill is:

a.)6,000

b.)10,000

c,)8,000

d.)none of the above

This is a question in CPT material sent from ICAI. The answer was given as A i.e. 6000. please explain how to solve this problem