[danadam]

From 33 to 51 is 3 steps up in the test. If for example you can consistently make it to 33 by ear and then randomly choose A or B from there on, on average once in 8 (=2^3) complete tests you make it to 51.

No, those 2 repeated steps at 51 are not optional, so it's only 1 in 32 chance.