To solve, you simply must multiply 15 by 0.8. You multiply by 0.8 and not 0.2 because you are calculating 80% of the original cost, which is the same thing as 20% off.
15 x 0.8 = 12
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If you take 12 and multiply it by 10 because that is the monthly subscription, you will get 120 + 60 for the game which equals 180. But if you take 12 and multiply it by 15 you will also get 180.