If 1/(b – a) + 1/(b – c) = 1/a + 1/c, then a, b, c form a/an

Mnonopqopqrs- - - - -

P, Q, R, S and T are playing a game of cards. P says to Q, "If you give me six cards, you will have as many cards as T has and if I give you three cards, you will have as many cards as S has." P and Q together have 20 cards more than what S and T together have. If Q has two cards more than what R has and the total number of cards be 133, how many cards does Q have?