dalthor, 146 is correct:
You need to buy eight challenges.
9+12+15+18+21+24+27+30 = 156.
Of your three original ones, the difficult remains unsolved. So you get 1+2 = 3 from playing the original easy and medium, then seven more for playing 7 easy purchases.
Return of 10, so 156-10 = 146.
(The discrepancy is caused by you not being guaranteed to solve a difficult in the 8-buy case).
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