You know you're keeping the bike for three days. So Plan A is $60 + .12x, where x is the number of miles, and Plan B is $75 + .06x.
The question is when is B less than A.
75+.06x < 60+.12x. Solve it for x, the number of miles.
15 < .06x
250 < x
Plan B costs less when you ride more than 250 miles.
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