How could I make a passive income with a bank of 25 of the cheapest new smartphones available? What apps can I let them just run all day and night and have their processing power earn me money?

If you had 25 spare smartphones, how would you make money with their processing power?

What apps could you run that would let them make money just for running those apps?

How long would the ROI of these phones take?

And how do I make them connect to the web without paying extra for extra connections/service?