Answer:
36 minutes
Explanation:
Flat fee = $12
Charges per minutes = $0.25
Total charges = $21
Total charges = flat fee + charges per minutes
21 = 12 + 0.25x
21 - 12 = 0.25x
9 = 0.25x
x = 9 / 0.25
= 36
x = 36 minutes
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