  Slot machines can have fairly complex behaviour. Determining the RTP (return
to player) can be involved, especially when a player has an influence on the
course of the game. In this paper we model the behaviour of slot machines using
probabilistic process specifications where the intervention of players is
modelled using non-determinism. The RTP is formulated as a quantitative modal
formula which can be evaluated fully automatically on the behavioural
specifications of these slot machines. We apply the method on an actual slot
machine provided by the company Err\`el Industries B.V. The most useful
contribution of this paper is that we show how to describe the behaviour of
slot machines both concisely and unequivocally. Using quantitative modal logics
there is an extra bonus, as we can quite easily provide valuable insights by
a.o. computing the exact RTP and obtaining the optimal player strategies.
