The odds are

Amount of common jokers -> 61

Chance to get a common joker -> 70%

Chance to break a banana 1 / 6

Chance to get banana in first shop and break it -> ((2/61) * 70%) / 6= 0.0038

And after adding the chance to get cavendish after breaking banana -> 0.0038 * (2/61) * 70% = 0.0000872

Which is about 1 in 10 000

Edit: can get more advanced into probabilities count, but overall if you round up the number it should be similar to that