Answer:
The inequality to set up is

It solves to

Bill needs to run at least 15 more miles.
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Step-by-step explanation:
He has run 16 miles so far. Add on another t miles to get 16+t or t+16 to represent the total amount he runs. This expression must be 31 or larger
Either t+16 > 31 or t+16 = 31
Those two items condense to

To solve for t, we subtract 16 from both sides to undo the +16.

Bill needs to run at least 15 more miles.
Meaning that t = 15, t = 16, t = 17, etc.