Python 3.4.3 test bugged:

'NO' should equal 'YES'
[25, 100]

Sell 1st ticket, get 25 (total in hand 25)
Change to give: 0
Sell 2nd ticket, get 25 (total in hand 50)
Change to give: 75 (vs 50 in hand)

There are other examples like this, but this is the simplest. The tests need looking at.

Furthermore, this directly contradicts the second example test given:
tickets([25, 100]) # => NO. Vasya will not have enough money to give change to 100 dollars