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Answer:
Step-by-step explanation:
The cost of the AT&T plan for m messages is ...
 a(m) = 25 +0.05m
The cost of the Verizon plan for m messages is ...
 v(m) = 15 +0.10m
The two plans have equal costs when ...
 a(m) = v(m)
 25 + 0.05m = 15 + 0.10m
 10 = 0.05m . . . . . . . . . . . . . . subtract 15+0.05 m from both sides
 200 = m . . . . . . . . . multiply both sides by 20
The plans have the same cost for 200 messages.
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For fewer than 200 messages, the plan with the lower fixed fee is the better choice.
Teresa should choose the Verizon plan.