Answer:
You could talk on the phone for 200 minutes at night, or 40 minutes during the day.
For every 5 minutes you reduced your night time talking, you could talk during the day for 1 minute.
Step-by-step explanation:
0.50d + 0.10n < $20
0.50d < $20 --> d < 40 minutes
0.10n < $20 --> n < 200 minutes
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