According to listings on Amazon as of April 2019, one can get an Antminer S9 for $349 with free shipping, plus the APW3 power supply for it for $88. Maybe this is a good deal! Let's see.

The Antminer S9 can mine at 13 THPS (tera hashes per second), and consumes 1300W of power.

The current bitcoin mining hash target requires 19 hex leading zeros. You pay 15 cents per KWh of power.

Bitcoin mining reward is 12.5 BTC per block, and bitcoin is trading at $5000 USD.

What is your daily profit?

In the previous problem, imagine your marginal power costs are zero (perhaps you live in a dorm or apartment where electricity is included in rent). In that case, how long will it take (in days) to make your money back on the purchase of the Antminer S9 + PSU?